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\DOI{10.5802/crchim.439}
\datereceived{2025-06-03}
\daterevised{2025-12-03}
\dateaccepted{2025-12-17}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliations other than their research organizations.}

\dateposted{2026-06-03}
\begin{document}

%\dateposted{2026-02-16}

\begin{noXML}

\editornote{Submitted by invitation as part of the 13th edition of the Moroccan Meetings on Solid State Chemistry (REMCES-13).}
\alteditornote{Soumis dans le cadre de la 13\textsuperscript{\`eme} \'edition des Rencontres Marocaines sur la Chimie de l'\'Etat Solide (REMCES-13).}

\CDRsetmeta{articletype}{research-article}

\title{Study and characterization of iron orthophosphate 
BaSb\textsubscript{0.5}Fe\textsubscript{0.5}(PO\textsubscript{4})\textsubscript{2}}

\alttitle{\'Etude et caract\'erisation de l'orthophosphate de fer
BaSb\textsubscript{0.5}Fe\textsubscript{0.5}(PO\textsubscript{4})\textsubscript{2}}

\author{\firstname{Fatima Zahra} \lastname{Tabane}\IsCorresp}
\address{Laboratory of G\'enie des Mat\'eriaux, 
des Proc\'ed\'es et de l'Environnement (GeMaPE), 
Faculty of Sciences Ain Chock, Hassan II University, Casablanca, Morocco}
\email[F. Z. Tabane]{tabanefatimazahra@gmail.com}

\author{\firstname{Rachid} \lastname{Fakhreddine}\CDRorcid{0000-0002-4741-9509}}
\addressSameAs{1}{Laboratory of G\'enie des Mat\'eriaux, 
des Proc\'ed\'es et de l'Environnement (GeMaPE), 
Faculty of Sciences Ain Chock, Hassan II University, Casablanca, Morocco}

\author{\firstname{Hajar} \lastname{Bellefqih}\CDRorcid{0009-0004-8510-6986}}
\address{Ecole Nationale Sup\'erieure des Mines de Saint Etienne, 
CNRS UMR, EVS, 5600, F42023 Saint Etienne, France}

\author{\firstname{Aziz} \lastname{Zaroual}\CDRorcid{0009-0002-2856-699X}}
\address{Laboratory of Materials Nanotechnology and Environment, 
Faculty of Sciences Rabat, Mohamed V University, Rabat, Morocco}

\author{\firstname{Saida}\nobreakauthor\lastname{Krimi}\CDRorcid{0000-0001-5528-7991}}
\addressSameAs{1}{Laboratory of G\'enie des Mat\'eriaux, 
des Proc\'ed\'es et de l'Environnement (GeMaPE), 
Faculty of Sciences Ain Chock, Hassan II University, Casablanca, Morocco}

\author{\firstname{Abderrahim} \lastname{Aatiq}}
\address{Faculty of Sciences Ben Sick, 
University Hassan II of Casablanca, Morocco}

\shortrunauthors

\keywords{\kwd{Iron orthophosphate}
\kwd{Solid-state reaction}
\kwd{Scanning electron microscopy}
\kwd{Optoelectronics}}

\altkeywords{\kwd{Orthophosphate de fer}
\kwd{R\'eaction \`a l'\'etat solide}
\kwd{Microscopie \'electronique \`a balayage}
\kwd{Opto\'electronique}}

\begin{abstract}
In  this paper, we report the synthesis, structural, vibrational, and
optical characterization  of the BaSb\textsubscript{0.5}Fe\textsubscript{0.5}(PO\textsubscript{4})\textsubscript{2}
compound, prepared via the  conventional solid-state reaction method.
X-ray powder diffraction analysis, followed  by Rietveld refinement,
revealed that the material crystallizes in the monoclinic  system with
the \textit{C}2/m space group and  \textit{Z} = 2, consistent with a
yavapaiite-type  structure. Scanning electron microscopy was employed
to examine the microstructure  and confirm elemental homogeneity. Raman
and infrared spectroscopy were used to investigate  the internal modes
of the phosphate units, providing evidence for well-defined
\tralicstex{PO\textsubscript{4}\textsuperscript{3−}}{$\mathrm{PO}_{4}^{3-}$}  tetrahedra and confirming the integrity of the crystal
framework. UV--visible spectroscopy  measurements indicated both direct
and indirect optical band gap energies of 2.96  and 3.28 eV,
respectively, suggesting that BaSb\textsubscript{0.5}Fe\textsubscript{0.5}(PO\textsubscript{4})\textsubscript{2}
behaves  as a semiconducting material. The combination of a
well-ordered crystal structure,  vibrational stability, and
semiconducting properties highlights the potential of  this
phosphate-based compound for applications in optoelectronic devices and
functional  materials.
\end{abstract}

\begin{altabstract}
Dans ce travail, nous rapportons la synth\`ese ainsi que la
caract\'erisation  structurale, vibrationnelle et optique du compos\'e
BaSb\textsubscript{0.5}Fe\textsubscript{0.5}(PO\textsubscript{4})\textsubscript{2},  pr\'epar\'e par la m\'ethode
conventionnelle de r\'eaction \`a l'\'etat solide. La caract\'erisation
structurale par diffraction des rayons X en utilisant la m\'ethode
d'affinement Rietveld,  montre que le mat\'eriau cristallise dans une
structure de type yavapaiite (syst\`eme  monoclinique, avec le groupe
d'espace \textit{C}2/m et \textit{Z} = 2). La microscopie  \'electronique \`a
balayage a \'et\'e utilis\'ee pour examiner la microstructure et
v\'erifier  l'homog\'en\'eit\'e \'el\'ementaire. Les spectroscopies
Raman et infrarouge ont permis d'analyser  les modes vibrationnels
internes des groupements phosphate, r\'ev\'elant des t\'etra\`edres 
PO\textsubscript{4}\textsuperscript{3−} bien d\'efinis et confirmant la stabilit\'e structurale.
Les mesures de  spectroscopie UV--visible indiquent des bandes optiques
directe et indirecte de 2,96  et 3,28 eV respectivement, d\'emontrant
que BaSb\textsubscript{0.5}Fe\textsubscript{0.5}(PO\textsubscript{4})\textsubscript{2} pr\'esente  un comportement
semi-conducteur. La combinaison d'une structure cristalline bien 
ordonn\'ee, d'une stabilit\'e vibrationnelle et de propri\'et\'es
semi-conductrices met en  \'evidence le potentiel de ce mat\'eriau
phosphat\'e pour des applications dans les dispositifs 
opto\'electroniques et les mat\'eriaux fonctionnels avanc\'es. 
\end{altabstract}

%\input{CR-pagedemetas}

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\section{Introduction}\label{sec1}
In recent years, phosphate-based compounds have garnered considerable
attention  within the scientific community owing to their intrinsic
structural diversity and  the breadth of their potential technological
applications. The fundamental building  units of these materials,
PO$_{4}$ tetrahedra, \mbox{enable} the \mbox{construction} of a wide  array of
crystalline \mbox{architectures,} \mbox{allowing} the systematic
modulation of physical  and chemical properties through compositional
and structural variation. This versatility  has rendered phosphate
frameworks especially attractive for use in fields such as  energy
storage, catalysis, ion exchange, and solid-state ion conduction.

Within this broad class of materials, transition-metal phosphates have 
emerged as particularly promising candidates. Their utility stems from
a unique combination  of chemical robustness, \mbox{redox} flexibility, and
tunable framework geometries. Several  structural families have been
extensively studied for their performance in various  technological
domains. Among them, phosphates of the NASICON (sodium super ionic 
conductor) type have drawn significant interest due to their
three-dimensional open-framework  structures and exceptional ionic
conductivity, which are ideal for lithium- and sodium-ion  battery
applications~\cite{1,2}. Likewise, diphosphates and langbeinite-type
phosphates  have been explored for their thermal stability and
catalytic efficiency in both environmental  and industrial
processes~\cite{3,4,5}. Monophosphates, which are structurally simpler
yet  chemically versatile, have shown promise in electrochemical
devices and ion transport  systems~\cite{6,7,8,9,10}.

A notable subcategory within monophosphates comprises compounds with
general  formula A$^{\mathrm{II}}$M$^{\mathrm{IV}}$(PO$_{4}$)$_{2}$, 
where A$^{\mathrm{II}}$ represents a divalent  cation (e.g., Ca$^{2+}$,
Sr$^{2+}$,  Ba$^{2+}$) and M$^{\mathrm{IV}}$ denotes a tetravalent
cation (e.g., Zr$^{4+}$,  Sn$^{4+}$,  Ti$^{4+}$). These materials have
been the focus of several structural studies  aiming to correlate
chemical composition with crystallographic arrangement. Two primary 
structural types have been identified in this family: cheralite-type
and yavapaiite-type  frameworks. The distinction between these two
polymorphs is governed predominantly  by the relative sizes of the
constituent cations.

Specifically, a high ionic-radius ratio between the A$^{\mathrm{II}}$
and M$^{\mathrm{IV}}$ cations  favors the formation of the
yavapaiite-type structure, characterized by  an ordered network of
cationic polyhedra. This structural order is typically associated  with
enhanced material stability and predictable ion transport pathways,
which are  desirable for energy storage and ionic conduction
applications. In contrast, systems  with a lower radius ratio tend to
crystallize in the cheralite-type structure, which  exhibits a more
disordered arrangement of cations. While such disorder can be
detrimental  to certain properties such as ionic mobility it may, in
some contexts, introduce  defect-mediated functionalities that are
beneficial in applications like heterogeneous  catalysis or proton
\mbox{conduction~\cite{10}.}

These insights into the structure--composition relationships within
A$^{\mathrm{II}}$M$^{\mathrm{IV}}$(PO$_{4}$)$_{2}$ compounds  offer a
rational basis for the design and optimization of phosphate-based
materials.  By carefully selecting suitable cation pairs, it is
possible to direct the formation  of targeted structures with tailored
properties, thereby advancing the development  of high-performance
materials for next-generation electrochemical and catalytic
technologies.

Recently,  diverse compounds
$\mathrm{A}(\mathrm{M}_{0.5}^{\mathrm{V}}\mathrm{X}_{0.5}
^{\mathrm{III}})(\mathrm{PO}_{4})_{2}$ with 
M ${=}$ Sb, Nb; A ${=}$ Sr, Pb, Ba; and X ${=}$ Ga, Cr, Fe, Sc, In, Yb, Al
have been studied  in terms of structural and vibrational
properties~\cite{11,12,13,14,15,16,17,18}. The barium phases
$\mathrm{Ba}(\mathrm{X}_{0.5}^{\mathrm{III}}\mathrm{X}
_{0.5}^{\prime\mathrm{V}})(\mathrm{PO}_{4})_{2}$
crystallized in the monoclinic  $C2/\mathrm{m}$ space group  $(Z= 2)$, while
other phases  such as
$\mathrm{A}(\mathrm{Sb}_{0.5}\mathrm{X}_{0.5}^{\mathrm{III}}
)(\mathrm{PO}_{4})_{2}$ (A:~Pb, Sr; X:
Fe,  Cr, Ga)~\cite{11,13,14} crystallized in the distorted monoclinic
yavapaiite structure  type, of space group  $C2/c$ ($Z=4$). 

Searching UV--visible spectra of novel semiconducting phosphates, this
paper investigates  microstructural, vibrational properties of the
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ space  group 
$(C2/\mathrm{m},Z= 2)$~\cite{11}. In this context, microstructural morphology, 
Raman and infrared spectra and UV--visible absorption spectra are
studied for optical  properties of this phosphate.

\section{Experimental}\label{sec2} 

\subsection{Synthesis}\label{sec2.1} 
The BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ compound was prepared using
the  conventional high-temperature solid-state process from mixtures of
BaCO$_{3}$,  Fe$_{2}$O$_{3}$, Sb$_{2}$O$_{3}$, and
NH$_{4}$H$_{2}$PO$_{4}$ powders  in an appropriate stoichiometric
ratio.  The reagents were transferred to an agate  mortar and finely
ground into a perfectly homogeneous mixture, which was then heated  in
air to a final temperature of 930~\textdegree C. 

\subsection{Characterization techniques}\label{sec2.2} 
The X-ray powder diffraction (XRD) pattern of the resulting material
was  collected at room temperature using a D8 Advance Bruker
diffractometer equipped with  Cu anticathode (Cu--K$\upalpha $ radiation,
$\lambda = 1.5406$~\AA) and Lynx  Eye detectors. The measurements were
performed under Bragg--Brentano geometry at  $2\theta$ with
0.0105\textdegree\ steps in the 10\textdegree--80\textdegree\ range. Structural 
refinements were carried out using the FullProf Suite
\mbox{software~\cite{19}.}

Infrared spectra of the compounds were recorded using Bruker's VERTEX
70 FTIR spectrometer  in the 1500--400~cm$^{-1}$ range,
with the samples prepared as KBr pellets. Raman spectra  were recorded
on the RENISHAW 1000B spectrometer in the 50--1500~cm$^{-1}$ range. 

The microstructural surface features of the samples were examined using
scanning  electron microscopy (SEM) with a JEOL JSM-IT100 InTouchScope
microscope, operated  at 15~kV. Prior to imaging, the sample was
mounted on aluminum stubs using carbon  tape and coated with a thin
layer of carbon through metallization to ensure surface  conductivity
and to avoid charging effects. The observations were made at various 
magnifications (3000${\times}$ at 20$~\upmu$m and 2000${\times}$ at 
30~$\upmu$m). 

\section{Results and discussion}\label{sec3}

\subsection{Structure of BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$
phase}\label{sec3.1} 
The Rietveld study, performed using the Le Bail profile, revealed that
the BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ phase  crystallizes in the
monoclinic  $C2/\mathrm{m}$ space group. Accordingly, the initial  structural
parameters for the Rietveld refinement were adopted from the previously
reported BaSb$_{0.5}$Ga$_{0.5}$(PO$_{4}$)$_{2}$ phase by Fakhreddine
et~al.~\cite{14},  which also belongs to the  $C2/\mathrm{m}$ space group. The
refinement yielded  satisfactory reliability indicators, with 
$R_{\mathrm{F}}=
1.6\%$,  $R_{\mathrm{B}}=  2.2\%$, and $\chi^2= 1.3$. The Rietveld refinement
for the BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ compound  is illustrated
in Figure~\ref{fig1}, while the detailed refinement parameters are
summarized  in 
\mbox{Table~\ref{tab1}}. Table~\ref{tab2} presents the XRD data
obtained from the ``observed intensities''  of the Rietveld refinement
(Cu--K$_{\upalpha1}$; $\lambda=1.5406$~\AA).

\begin{figure} 
\includegraphics{fig01}
\caption{\label{fig1}Rietveld refinement of the
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ phase.} 
\vspace*{-2.5pt}
\end{figure}

\begin{table*}
\caption{\label{tab1}Crystallographic data for
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$}
\tabcolsep=7.5pt
\begin{tabular}{ccccccc}
\tbody
\multicolumn{7}{l}{\parbox[t]{12.1cm}{\raggedright Space group
$C2/\mathrm{m}$ (N\textdegree\ 12); [$Z=2$;  $a =8.1661$ (4)~\AA; $b=
5.1934$ (3)~\AA; $c=7.8212$ (4)~\AA;  $\beta= 94.51$ (1)\textdegree;
$V=331$ (1)~\AA$^{3}$]}}\\
\multicolumn{7}{l}{Experimental data}\\
\multicolumn{7}{l}{Temperature 25~\textdegree C; 
angular range $10\mbox{\textdegree}\ \leq2\theta\leq80\mbox{\textdegree}$}\\
\multicolumn{7}{l}{step scan increment $(2\theta)$: 0.0105\textdegree}\\
\multicolumn{7}{l}{Zero point $(2\theta)$, ${-}$0.021 (1)\textdegree}\\
\multicolumn{7}{l}{Profile parameters}\\
\multicolumn{7}{l}{Pseudo-Voigt function, $\mathrm{PV} = 
\eta \mathrm{L} + (1-\eta)\mathrm{G}$; 
$\eta=0.586$ (2)}\\
\multicolumn{7}{l}{Half-width parameters, $U=0.369$ (3), $V=-0.0597$ (4), 
and $W=0.0120$ (1)}\\
\multicolumn{7}{l}{Conventional Rietveld  $R$-factors, 
$R_{\mathrm{WP}}=6.7\%$; $R_{\mathrm{P}}=4.9\%$; $R_{\mathrm{B}}=2.20\%$; 
$R_{\mathrm{F}}=1.59\%$}
\vspace*{4pt}\\
\xxbotline
Atom & Site & \multicolumn{3}{c}{Wyckoff positions} & 
$B_{\mathrm{iso}}$ (\AA$^{2}$) & Occupancy \\
\xxbotline
Ba &      2c & 0 &          0 &           0.5 &        0.7 (1) & 1\\
(Fe/Sb) & 2a & 0 &          0 &           0 &          0.3 (2) & 0.5/0.5\\
P &       4i & 0.3644 (1) & 0 &           0.2045 (1) & 0.4 (1) & 1\\
O (1) &   4i & 0.2342 (3) & 0 &           0.0560 (2) & 0.7 (4) & 1\\
O (2) &   4i & 0.3106 (2) & 0 &           0.3854 (2) & 0.4 (3) & 1\\
O (3) &   8j & 0.4839 (3) & 0.2414 (2) &  0.1922 (1) & 0.6 (2) & 1
\botline
\end{tabular}
\end{table*}

\begin{table*}
\caption{\label{tab2}Powder  diffraction data of
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ (Cu--K$_{\upalpha1}$;
$\lambda=1.5406$~\AA)}
\begin{tabular}{cccccccc}
\thead
$hkl$ & $d_{\mathrm{obs}}$~(\AA) & 
\parbox[t]{1.2cm}{\centering $100~I/I_{0}$\ (obsd)} & 
\parbox[t]{1.2cm}{\centering$100~I/I_{0}$\ (calcd)} & 
$hkl$ & $d_{\mathrm{obs}}$~(\AA) & 
\parbox[t]{1.2cm}{\centering$100~I/I_{0}$\ (obsd)} &
\parbox[t]{1.2cm}{\centering$100~I/I_{0}$\ (calcd)}\vspace*{2.5pt} \\
\endthead
\mn001 & 7.7970 & \0\05 &  \0\05 & \mn132 & 1.5449 & 6 & 5\\
\mn110 & 4.3783 & 100 &    100 &   ${-}$422 & 1.5150 & 6 & 5\\
\mn200 & 4.0704 & \064 &   \068 &  ${-}$224 & 1.4887 & 5 & 4\\
\mn002 & 3.8985 & \034 &   \032 &  ${-}$512 & 1.4811 & 3 & 3\\
${-}$111 & 3.8884 & \015 &   \016 &  \mn314 & 1.4654 & 4 & 5\\
\mn111 & 3.7506 & \039 &   \039 &  ${-}$404 & 1.4667 & 4 & 4\\
${-}$201 & 3.7306 & \022 &   \021 &  \mn330 & 1.4602 & 7 & 6\\
\mn201 & 3.4944 & \0\03 &  \0\04 & \mn422 & 1.4502 & 5 & 6\\
${-}$112 & 2.9747 & \092 &   \090 &  \mn224 & 1.4271 & 2 & 1\\
${-}$202 & 2.9330 & \010 &   \010 &  \mn512 & 1.4063 & 2 & 3\\
\mn112 & 2.8523 & \052 &   \053 &  ${-}$332 & 1.3871 & 1 & 1\\
\mn202 & 2.7111 & \0\09 &  \010 &  \mn600 & 1.3552 & 2 & 2\\
\mn020 & 2.5990 & \031 &   \029 &  \mn332 & 1.3489 & 1 & 1\\
\mn310 & 2.4051 & \034 &   \036 &  \mn040 & 1.3000 & 3 & 3\\
${-}$311 & 2.3447 & \0\01 &  \0\01 & ${-}$134 & 1.2902 & 2 & 2\\
${-}$203 & 2.2776 & \0\03 &  \0\02 & \mn134 & 1.2693 & 2 & 2\\
\mn311 & 2.2544 & \0\02 &  \0\02 & ${-}$116 & 1.2619 & 5 & 4\\
\mn220 & 2.1945 & \0\05 &  \0\04 & \mn240 & 1.2372 & 1 & 1\\
\mn022 & 2.1612 & \018 &   \017 &  \mn042 & 1.2323 & 1 & 1\\
${-}$221 & 2.1312 & \0\07 &  \0\06 & \mn206 & 1.2108 & 2 & 1\\
${-}$312 & 2.1137 & \014 &   \012 &  \mn424 & 1.2016 & 3 & 3\\
\mn400 & 2.0352 & \0\05 &  \0\06 & & & &\\
\mn312 & 1.9860 & \0\06 &  \0\06 & & & &\\
\mn004 & 1.9493 & \016 &   \014 &  & & &\\
\mn222 & 1.8753 & \0\09 &  \0\09 & & & &\\
${-}$402 & 1.8653 & \0\04 &  \0\04 & & & &\\
${-}$313 & 1.8299 & \0\03 &  \0\03 & & & &\\
${-}$204 & 1.8159 & \0\09 &  \0\09 & & & &\\
${-}$114 & 1.8144 & \011 &   \010 &  & & &\\
\mn114 & 1.7534 & \0\07 &  \0\07 & & & &\\
\mn402 & 1.7479 & \0\06 &  \0\09 & & & &\\
\mn130 & 1.6933 & \0\04 &  \0\03 & & & &\\
\mn131 & 1.6491 & \0\02 &  \0\02 & & & &\\
\mn420 & 1.6018 & \0\03 &  \0\04 & & & &\\
${-}$132 & 1.5625 & \010 &   \0\09 & & & &\\
\mn510 & 1.5536 & \0\05 &  \0\05 & & & &
\botline
\end{tabular}
\end{table*}

The three-dimensional framework of this phosphate can be considered as
composed of an  alternation of BaO$_{10}$ polyhedra, PO$_{4}$
tetrahedra, and Sb(Fe)O$_6$ octahedra.\ The BaO$_{10}$ polyhedra and
Sb(Fe)O$_{6}$ octahedra share edges and form infinite  chains parallel
to the c-axis. Each Sb(Fe)O$_{6}$ octahedron is connected at its 
vertices to six PO$_{4}$ tetrahedral groups. The PO$_{4}$ tetrahedra
are isolated,  and each is linked, through two of its edges, to two
BaO$_{10}$ polyhedra. The  three-dimensional framework consists of
layers of Ba$^{2+}$ cations in tenfold coordination,  alternating with
dense layers made up of MO$_{4}$ 
\mbox{octahedra} (M ${=}$ Sb, Fe) and PO$_{4}$
tetrahedra,  which are interconnected via their vertices 
(Figures~\ref{fig2} and~\ref{fig3}).

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}Structure of the
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ phase.}
\vspace*{-2.5pt}
\end{figure}

\begin{figure}
\includegraphics{fig03}
\vspace*{5pt}
\caption{\label{fig3}Projection onto the $(ac)$ plane of the structure 
of the BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ phosphate.}
\end{figure}

\subsection{Scanning electron microscopy (SEM)}\label{sec3.2}
SEM Investigation by SEM of the BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$
powder  (Figure~\ref{fig4}), recorded at 2000${\times}$ and 3000${\times}$
magnifications, revealed a  heterogeneous microstructure consisting of
agglomerated grains of various sizes,  interspersed with intergranular
pores. The grains exhibit irregular morphology and  clustering
behavior, indicative of polycrystalline growth with incomplete
densification,  a common feature in materials synthesized at moderate
temperatures. Based on the  scale bars (30~$\upmu$m and 20~$\upmu$m),
the individual grains and clusters display approximate  sizes ranging
from 2 to 10~$\upmu$m, with some larger \mbox{agglomerates} extending
beyond 10~$\upmu$m. Such a morphology could influence the material's
functional properties, particularly  in relation to surface activity
and ionic diffusion paths. \looseness=-1

\begin{figure} 
\includegraphics{fig04}
\caption{\label{fig4}SEM micrographs of
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ at  2000${\times}$ (a) and
3000${\times}$ (b) magnifications.}
\end{figure}

To evaluate the elemental composition of the synthesized material and
verify  the target stoichiometry, energy-dispersive X-ray spectroscopy
(EDS) was performed.  The quantitative results obtained from the EDS
spectra (Figure~\ref{fig5} and Table~\ref{tab3}) provide  the mass and
atomic percentages of the elements Ba, Fe, Sb, P, and O in the compound
studied. From the atomic percentages, the calculated ratios (Ba/P ${=}$
0.557, Fe/P ${=}$  0.284, Sb/P ${=}$ 0.307, Ba/Sb ${=}$ 1.82, and Ba/Fe ${=}$
1.96) align well with the theoretical  values for the
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ composition (Ba/P ${=}$  0.50,
Fe/P ${=}$ 0.25, Sb/P ${=}$ 0.25, Ba/Sb ${=}$ 2.00, and Ba/Fe ${=}$ 2.00). The
small deviations  observed are typical of EDS quantification due to
matrix effects and the limited  sensitivity for light elements.
Nevertheless, the overall consistency between experimental  and
theoretical ratios confirms that the targeted stoichiometry is
achieved, indicating  successful incorporation of Sb and Fe in the
structure. This result strongly supports  the homogeneity of the
sample, and the 
\mbox{effectiveness} of the synthesis route used  for
producing a well-defined, single-phase phosphate compound.

\begin{figure*}
\includegraphics{fig05}
\vspace*{4pt}
\caption{\label{fig5}EDX  spectrum of
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$.}
\vspace*{12pt}
\end{figure*}

\begin{table}
\caption{\label{tab3}EDX composition analysis of 
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$}
\begin{tabular}{ccc}
\thead
Element & Weight (\%) & Atom (\%) \\
\endthead
{C} & \015.0 & \032.0 \\
{O} & \030.6 & \049.2 \\
{P} & \010.6 & \0\08.8 \\ 
{Fe} & \0\05.2 & \0\02.5 \\ 
{Sb} & \012.6 & \0\02.7 \\
{Ba} & \026.0 & \0\04.9 \\
{Total} & 100.0 & 100.0
\botline
\end{tabular}
\end{table}

\subsection{Infrared and Raman study}\label{sec3.3}
Raman and infrared spectroscopic analyses were carried out to obtain a 
deeper understanding of the bonding environment within the
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ compound.  Given that its
structure consists of both isolated PO$_{4}$ tetrahedra and
Sb(Fe)O$_{6}$ octahedra,  its vibrational features are characteristic
of orthophosphates. The vibrational behavior  of PO$_{4}$ tetrahedral
units is well established, and in such structures, the  vibrational
modes of the phosphate groups typically dominate over lattice
vibrations  and metal--oxygen interactions. Raman and IR spectra of the
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ phase  are given in 
Figures~\ref{fig6} and~\ref{fig7}. The vibrational behavior of the
phosphate units in  the BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ compound
was investigated through  infrared and Raman spectroscopic techniques.
The analysis revealed distinct vibrational  bands corresponding to the
internal modes of the $\mathrm{PO}_{4}^{3-}$ tetrahedra,  consistent with
expectations for well-ordered phosphate frameworks.

\begin{figure}
\includegraphics{fig06}
\vspace*{4pt}
\caption{\label{fig6}Infrared spectrum of
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$.}
\end{figure}

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}Raman spectrum of 
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$.}
\end{figure}

The symmetric non-degenerate stretching mode $(\nu_1)$ of the P--O bond
was clearly observed in the 918--1023~cm$^{-1}$ spectral range, which
is  typical for isolated tetrahedral phosphate groups. This band is
usually the most  intense in infrared spectra and serves as a strong
indicator of the presence of PO$_{4}$ units.

In  the low-frequency region between 429 and 485~cm$^{-1}$, vibrational
features were  identified that correspond to the antisymmetric doubly
degenerate bending modes $(\nu_2)$  of the PO$_{4}$ tetrahedra. These
modes are sensitive to distortions within the  tetrahedron and can
reflect subtle differences in the local structural environment  around
the phosphate group.

The high-frequency region ranging from 1050 to  1157~cm$^{-1}$
exhibited  bands attributed to the triply degenerate O--P--O asymmetric
stretching modes  $(\nu_3)$.  The presence of these modes further
confirms the integrity of the phosphate framework  and the preservation
of tetrahedral symmetry, albeit slightly distorted due to cation 
substitution.

Additionally, the bands detected in the 526--657~cm$^{-1}$ region were 
assigned to the harmonic bending vibrations of O--P--O linkages 
$(\nu_4)$. These  bending modes are characteristic of internal
deformations within the PO$_{4}$ group  and are commonly used to
evaluate the degree of distortion in the tetrahedral geometry.

Altogether,  the vibrational spectra confirm the presence and stability
of the phosphate units  within the crystal structure and support the
assignment of BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ to  the
yavapaiite-type framework, where the PO$_{4}$ groups retain a nearly
ideal  tetrahedral configuration, as commonly observed in yavapaiite
phosphates~\cite{12,13,14,15,16,17,18}.

In  the lattice-mode region of Raman spectra (400--50~cm$^{-1}$), the
translational  modes of $\mathrm{PO}_{4}^{3-}$ ions as well as vibrational modes
of $\mathrm{PO}_{4}^{3-}$ ions  and BaO$_{10}$ groups should be expected. At
wavenumbers below 400~cm$^{-1}$,  coupling occurs between the different
bending vibrations O--P--O, O--Ba--O, Ba--O--P.  The strong Raman bands
observed at 360~cm$^{-1}$ may be assigned to Fe--O stretching 
vibration modes~\cite{20}. The low-frequency modes observed below
284~cm$^{-1}$ can  be easily attributed to translational modes of the
Ba$^{2+}$, Fe$^{3+}$, Sb$^{5+}$, and  $\mathrm{PO}_{4}^{3-}$ ions.

\subsection{UV--visible  analysis}\label{sec3.4} 
The optical properties of BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ were 
studied at room temperature by measuring UV--visible absorption spectra
(200--800  nm) shown in  Figure~\ref{fig8}. The UV--Vis absorption
spectrum of the compound reveals a  pronounced and broad absorption
edge in the ultraviolet region (between 200 and 300~nm).
This band is mainly attributed  to ligand-to-metal charge transfer
transitions, primarily  $\mathrm{O}^{2-}\rightarrow  \mathrm{Fe}^{3+}$,
which is the main optically active center in the structure. A minor
contribution  from  $\mathrm{O}^{2-}\rightarrow \mathrm{Sb}^{5+}$
transitions cannot be excluded~\cite{14,15,16,17,21}.  The optical
spectrum of BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ also shows  three
low-intensity bands centered around 750, 520, and 417~nm. According to
the   literature, these bands are attributed, respectively, to the
forbidden transitions $\mbox{}^{6}$A$_{1\mathrm{g}}$ (S)
$\rightarrow$\ $\mbox{}^{4}$T$_{2\mathrm{g}}$ (G) (750~nm),
$\mbox{}^{6}$A$_{1\mathrm{g}}$  (S) $\rightarrow$\ 
$\mbox{}^{4}$T$_{1\mathrm{g}}$ (G)  (520~nm), and
$\mbox{}^{6}$A$_{1\mathrm{g}}$ (S) $\rightarrow$\ 
$\mbox{}^{4}$A$_{1\mathrm{g}}$ (G),  $\mbox{}^{4}$E$_{2\mathrm{g}}$ (G)
(417~nm) of the Fe$^{3+}$ ions in octahedral sites~\cite{22}.

\begin{figure}
\includegraphics{fig08}
\vspace*{4pt}
\caption{\label{fig8}UV--visible absorption spectrum of 
BaSb$_{0.5}${\ubreak}Fe$_{0.5}$(PO$_{4}$)$_{2}$.}
\end{figure}

The direct or optical indirect band gap energies  $E_{\mathrm{g}}$ can be
calculated  for the salts using the Tauc equation 
(Equation~(\ref{eq1}))~\cite{23}. The absorption coefficient  $\alpha $
is  determined using  Equation~(\ref{eq2})~\cite{24}, where  $A$ and
$d$ indicate the absorbance   and thickness of the cuvette used,
respectively.
{\begin{eqnarray}
\alpha h\nu  &=& B(h\nu - E_{\mathrm{g}})^{m} \label{eq1}\Seqnsplit
\alpha &=& (2.3003A)/d \label{eq2}
\end{eqnarray}}\unskip
where  $h$ is the Planck constant,  $\nu$ the photon frequency,  $B$ a 
constant, and  $m$ a constant determining the optical transition type.

Figures~\ref{fig9} and~\ref{fig10} show $(\alpha h\nu)^{2}$ and 
$(\alpha h\nu )^{1/2}$ variation  versus  $h\nu$ for
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$. The direct  energy gap is
estimated at  $E_{\mathrm{g}}= 2.96$~eV and indirect at $E_{\mathrm{g}}
=  3.28$~eV.  As found elsewhere, band gaps estimated at 4.16~eV for 
BaZr(PO$_{4}$)$_{2}$ and  4.11~eV for 
Ba$_{0.97}$Zr(PO$_{4}$)$_{2}$:0.03Eu$^{3+}$ indicate a red shift  in
the optical band gap, which is attributed to the dopant  Eu$^{3+}$ ion
generating  intermediate energy levels around valence and conduction
bands, leading to band gap narrowing~\cite{25}. Generally, the values
obtained for gap energies indicate a semiconductor  character.

\begin{figure}
\includegraphics{fig09}
\vspace*{4pt}
\caption{\label{fig9}Plot of $(\alpha h\nu )^{1/2}$ versus  $h\nu$ for 
BaSb$_{0.5}$Fe$_{0.5}${\ubreak}(PO$_{4}$)$_{2}$.}
\vspace*{10pt}
\end{figure}

\begin{figure}
\includegraphics{fig10}
\vspace*{4pt}
\caption{\label{fig10}Plot  of  $(\alpha h\nu)^{2}$ versus  $h\nu$ for
BaSb$_{0.5}$Fe$_{0.5}${\ubreak}(PO$_{4}$)$_{2}$.}
\end{figure}

\section{Conclusion}
In this study, the phosphate-based compound
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ was  synthesized and
structurally characterized. X-ray diffraction analysis confirmed  that
the compound crystallizes in the monoclinic yavapaiite-type structure,
assigned  to the $C2/\mathrm{m}$ space group with $Z = 2$, consistent with
structural trends observed  in other
A$^{\mathrm{II}}$M$^{\mathrm{IV}}$(PO$_{4}$)$_{2}$ systems where the
cationic  size ratio plays a pivotal role in stabilizing ordered
polyhedral frameworks.

The vibrational studies performed using infrared and Raman spectroscopy
confirmed the presence of characteristic $\mathrm{PO}_{4}^{3-}$ vibrational
modes. The spectral  features agree with previously reported data for
similar yavapaiite-type phosphates,  indicating the retention of
well-defined phosphate tetrahedra and supporting the  structural
assignment.

The optical absorption measurements revealed that
BaSb$_{0.5}$Fe$_{0.5}$(PO$_{4}$)$_{2}$ exhibits  a direct optical band
gap of 2.96~eV and an indirect band gap of 3.28~eV, classifying  it as
a wide band-gap semiconducting material. These electronic
characteristics suggest  potential applications in optoelectronic
devices, photocatalysis, or UV detection  technologies.

\printCOI

\back{}

\printbibliography
\refinput{crchim20250458-reference.tex}

\end{document}
