Electron paramagnetic resonance (EPR) study of solid solutions of MoO 3 in SbVO 5

1. Electron paramagnetic resonance (EPR) study of solid solutions ofMoO3 in SbVO5 Janusz Typek Institute of Physics West Pomeranian University of Technology Szczecin, Poland

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

3. 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

4. Components concentration triangle

5. Samples of MoO3 solid solutions in SbVO5 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). Preparation of samples V2O5+Sb2O4+1/2 O2→2 SbVO5 V2O5+Sb2O4+MoO3 Samples of SbVO5 were produced by heating in air equimolar mixture of V2O5 with α-Sb2O4 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)

6. Investigated samples General formulae of the solid solutions: Sb1-6x xV1-6xxMo10xO5

7. The matrix: SbVO5 Thickness ~0.5 μm Length ~3÷10 μm Scanning Electron Microscope (SEM) picture

8. The SbVO5 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: ● SbO6 octahedra ● VO6 deformed octahedra ● Separate layers

9. Solid solution SbVO5:MoO3 More deformed, smaller sizes SEM picture of SbVO5:MoO3 (15mol%)

10. SbVO5:MoO3 - Charge compensation • Preferred model of charge compensation, based on TG: • V5+ and Sb5+vacancies, • substitution of Mo6+ at V5+ and Sb5+ sites

11. EPR: paramagnetic centers Vanadium V5+ (3p6) nominal, nonmagnetic V4+ (3d1) defect, magnetic S=1/2 Antimony Sb5+ (4d10) nominal, nonmagnetic Sb4+ (5s1) defect, magnetic S=1/2 Molybdenum Mo6+ (4p6) nominal, nonmagnetic Mo5+ (4d1) defect, magnetic S=1/2

12. The SbVO5 matrix: EPR TCW=8 K I(T)=C/(T-TCW) • Only 0.02% of all vanadium ions are EPR active (V4+). • There are separate V4+ (showing 8 hfs narrow lines) and involved in a V4+–O–V5+ bond with mobile electron hopping (broad line). T=3.65 K D=19·10-4 cm-1 • Separate V4+ in SbVO5 exist as VO2+ ions in octahedral coordination with a tetragonal compression. • There are also pairs of two interacting VO2+ with a singlet S=0 (ground state) and a triplet S=1 state (excited state).

13. EPR: solid solution SbVO5:MoO3 • No hfs lines visible – all V4+ ions strongly coupled to the magnetic spin system. • The intensity of EPR spectra increases with the Mo6+ contents – only cation vacancy compensation model could not be used

14. EPR: solid solution SbVO5:MoO3 • No linear dependence of V4+ content on amount of Mo6+ ions. • The EPR linewidth decreases with Mo6+ content (exchange interaction narrowing). • The fraction of Mo6+ ions involved in V5+→V4+ compensation decreases with Mo6+ increase.

15. Solid solution: possible centres Possible paramagnetic Mo6+-V4+ centres involving one V4+ ion

16. Solid solution: possible centres Possible paramagnetic centres involving more than one V4+ ion(equatorial view)

17. Conclusions • At least one third of Mo6+ 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 Mo6+ ions • V4+ ions are strongly coupled to the rest of spin system – no distant charge compensation