Electron paramagnetic resonance (EPR) study of solid solutions of MoO 3 in SbVO 5 Janusz Typek Institute of Physics West Pomeranian University of Technology Szczecin, Poland

Outline The aim of this work Preparation and characterisation of samples Results of the EPR study – magnetic defects Conclusions

The aim of the work Why to study these materials? They are used widely as catalysts What are the oxidation states of ions? Only assumed on general grounds What is the structure of the defect centres? Not known

Components concentration triangle

Preparation of samples Samples of SbVO 5 were produced by heating in air equimolar mixture of V 2 O 5 with α-Sb 2 O 4 in the following stages: stage I: 550ºC→600ºC (48h), stage II: 600ºC→600ºC (48h); stage III: 600ºC→620ºC (24h); stage IV: 620ºC→650ºC (48h); stage V: 650ºC→650ºC (48h) Samples of MoO 3 solid solutions in SbVO 5 were made by homogenization of the reagents in suitable proportions by grinding, shaped into pastilles and then heated in the following stages: stage I 400ºC (1h)→500ºC (24h)→ 500ºC (24h); stage II: 600ºC (48h); stage III: 630ºC (24h); stage IV: 645ºC (24h). V 2 O 5 +Sb 2 O 4 +1/2 O 2 →2 SbVO 5 V 2 O 5 +Sb 2 O 4 +MoO 3

Investigated samples General formulae of the solid solutions: Sb 1-6x x V 1-6xx Mo 10x O 5 Composition of initial mixtures [%mol] Formulae index x MoO 3 V2O5V2O5 Sb 2 O

The matrix: SbVO 5 Scanning Electron Microscope (SEM) picture Thickness ~0.5 μm Length ~3÷10 μm

The SbVO 5 matrix: crystal structure Monoclinic a=9.86 Å, b=4.93 Å, c=7.12 Å, β=109.79°, Z=4 From IR study it follows that: ● SbO 6 octahedra ● VO 6 deformed octahedra ● Separate layers

Solid solution SbVO 5 :MoO 3 SEM picture of SbVO 5 :MoO 3 (15mol%) More deformed, smaller sizes

SbVO 5 :MoO 3 - Charge compensation Preferred model of charge compensation, based on TG: V 5+ and Sb 5+ vacancies, substitution of Mo 6+ at V 5+ and Sb 5+ sites

EPR: paramagnetic centers Vanadium V 5+ (3p 6 ) nominal, nonmagnetic V 4+ (3d 1 ) defect, magnetic S=1/2 Antimony Sb 5+ (4d 10 ) nominal, nonmagnetic Sb 4+ (5s 1 ) defect, magnetic S=1/2 Molybdenum Mo 6+ (4p 6 ) nominal, nonmagnetic Mo 5+ (4d 1 ) defect, magnetic S=1/2

The SbVO 5 matrix: EPR T=3.65 K D=19·10 -4 cm -1 Only 0.02% of all vanadium ions are EPR active (V 4+ ). There are separate V 4+ (showing 8 hfs narrow lines) and involved in a V 4+ –O–V 5+ bond with mobile electron hopping (broad line). Separate V 4+ in SbVO 5 exist as VO 2+ ions in octahedral coordination with a tetragonal compression. There are also pairs of two interacting VO 2+ with a singlet S=0 (ground state) and a triplet S=1 state (excited state). T CW =8 K I(T)=C/(T-T CW )

EPR: solid solution SbVO 5 :MoO 3 No hfs lines visible – all V 4+ ions strongly coupled to the magnetic spin system. The intensity of EPR spectra increases with the Mo 6+ contents – only cation vacancy compensation model could not be used

EPR: solid solution SbVO 5 :MoO 3 No linear dependence of V 4+ content on amount of Mo 6+ ions. The EPR linewidth decreases with Mo 6+ content (exchange interaction narrowing). The fraction of Mo 6+ ions involved in V 5+ →V 4+ compensation decreases with Mo 6+ increase.

Solid solution: possible centres Possible paramagnetic Mo 6+ -V 4+ centres involving one V 4+ ion

Solid solution: possible centres Possible paramagnetic centres involving more than one V 4+ ion (equatorial view)

Conclusions At least one third of Mo 6+ ions are involved in charge compensation through changing the oxidation state of cations Compensation mechanism through cation vacancy is more efficient for larger concentrations of Mo 6+ ions V 4+ ions are strongly coupled to the rest of spin system – no distant charge compensation