Variation of cation distribution with temperature and its consequences on thermal expansion for Ca₃Eu₂(BO₃)₄

Abstract

<jats:p>The structure of calcium europium orthoborate, Ca<jats:sub>3</jats:sub>Eu<jats:sub>2</jats:sub>(BO<jats:sub>3</jats:sub>)<jats:sub>4</jats:sub>, was determined using high-resolution powder X-ray diffraction data collected at the ID22 beamline (ESRF) under ambient conditions, as well as at high temperature. Rietveld refinement allowed determination of the lattice constants and structural details, including the Ca/Eu ratios at the three cationic sites and their evolution with temperature. Clear thermal expansion anisotropy was found, and slope changes of lattice-constant dependencies on temperature were observed at 923 K. Above this temperature the changes in occupation of the Ca/Eu sites occur, exhibiting a tendency towards a more uniform Eu distribution over the three Ca/Eu sites. Possible structural origins of the observed thermal expansion anisotropy are discussed.</jats:p>

Site-occupancy scheme in disordered Ca3RE2(BO3)4: a dependence on rare-earth (RE) ionic radius

“…Structure refinements were performed using TOPAS5 (Coelho, 2018) andFULLPROF (Rodríguez-Carvajal, 1993) software for synchrotron and laboratory data, respectively. The presented results include the data for RE = Eu from our recent article (Kosyl et al, 2020) in the present considerations as a part of the investigated series.…”

“…The synthesis of the Ca 3 RE 2 (BO 3 ) 4 borates turned out to be difficult (see x3.1); therefore, we applied several synthesis methods in order to obtain the desired compound. The samples selected for this article represent the best phase purity; syntheses of these samples were conducted using Czochralski growth [see Gudzenko et al (2017a) for growth details], the Pechini (1967) method (for synthesis details, see supporting information) and solid-state reaction (SSR) with or without consecutive melting of the product [for a description, see Kosyl et al (2020)]. All materials were investigated either on the ID22 beamline at European Synchrotron Radiation Facility in a high-resolution setup [wavelength used: = 0.400218 (4) Å ] or using a laboratory diffractometer (PANalytical X'Pert Pro MPD; Cu K 1 radiation).…”

“…Rietveld refinements (Fig. 1) were performed in the same manner as described in our recent article (Kosyl et al, 2020). The model used for the refinements was taken from the same article; accordingly, all the crystallographic site labels are consistent with the structure reported in this reference.…”

“…The M-based polyhedra are irregular; the range and distribution of M-O distances are specific for each of the M1, M2 and M3 sites, as was discussed in more detail in our previous work for the RE = Eu compound (Kosyl et al, 2020). Therefore, in order to compare the polyhedra for compounds with different rare earths, the average M-O distance for each polyhedron is presented in Fig.…”

“…For comparison, data from Bambauer & Kindermann (1978) are presented as crosses/pluses. Data for RE = Eu are taken from Kosyl et al (2020). Curves are guides for the eye.…”