Toward an understanding of the role of structural organization and point defects in the formation of functional pure and Tm3+-doped (Ca3-xSrx)(VO4)2 for diode-pumped and SRS lasers

• Published in: Solid state ionics Vol. 411; p. 116579
• Main Authors: Kuz'micheva, Galina M., Ivleva, Liudmila I., Voronina, Irina S., Kaurova, Irina A., Svetogorov, Roman D., Lazarenko, Vladimir A., Khramov, Evgeny V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-08-2024
• Subjects: Crystal growth; Local structure; Point defects; Solid solution; X-ray absorption spectroscopy; X-ray diffraction; Crystal growth; X-ray diffraction; Local structure; Point defects; X-ray absorption spectroscopy; Solid solution
• ISSN: 0167-2738; 1872-7689
• DOI: 10.1016/j.ssi.2024.116579

Whitlockite-type polycrystalline and single-crystal (Ca3−xSrx)(VO4)2 solid solutions were successfully obtained by solid-state synthesis and Czochralski method, respectively. Synchrotron powder X-ray diffraction allowed to reveal actual compositions of polycrystalline (Ca3−xSrx)(VO4)2 solid solutions with х = 0.3, 0.54, 0.9, 1.2, 1.35 refined by Rietveld method, which are slightly different from the initial ones. Synchrotron X-ray single-crystal diffraction analysis of Czochralski-grown undoped (Ca1.8Sr1.2)(VO4)2 (CSVO) revealed its actual composition (Ca2.982(11)Sr0.031)(VО4)2. Difference between initial and refined compositions is analyzed and discussed. In the CSVO structure, Sr2+ ions partially occupy monocapped Ca2 and two-capped Ca3 trigonal prismatic sites and Ca2+ ions are in the Ca5 and Ca5A sites, forming two independent strongly distorted tetrahedral Ca5O4 and octahedral Ca5AO6 oxygen environments. Synchrotron X-ray single-crystal diffraction analysis together with X-ray absorption spectroscopic study of (Сa2.4Sr0.6)(VO4)2 doped with 0.5 and 1.0 wt% Tm2O3 (СSVO:0.5Tm and CSVO:1.0Tm) allowed to establish the absence of cations in the Ca5A site, the localization of Sr2+ ions in the Ca2 and Ca3 sites and additionally in octahedral Ca4 site together with Tm3+ ions, forming an individual eight-coordinated environment, in the refined compositions (Ca2.776(7)Sr0.210(4)Tm3+0.008(1))(VO4)2 and (Ca2.855(7)Sr0.140(7)Tm3+0.009(1)□0.022(7))(VO4)2, respectively, with vacancies (marked by a square,□) in the two-capped Ca1 trigonal prismatic site in the CSVO:1.0Tm structure. Additional interstitial oxygen, participating in the formation of the Tm3+ environment, found in the residual electron density, together with vacancies in the Ca1 site provides electroneutrality to CSVO:0.5Tm and CSVO:1.0Tm crystals. Several structural models of the coordination environment of Tm3+ ions were analyzed and the most correct one was proposed.
•Polycrystalline and single-crystal (Ca3−xSrx)(VO4)2 with х = 0.3–1.35 are grown.•Sr2+ partially occupies monocapped Ca2 and two-capped Ca3 trigonal prismatic sites.•In Tm-doped (Ca3−xSrx)(VO4)2, Sr2+ and Tm3+ are located in octahedral Ca4 site.•A formal charge 3+ and 8-coordinated oxygen environment is established for Tm.•Additional interstitial oxygen and vacancies in the Ca1 site are found.