The influence of substitutional disorder on the structural phase transitions in Rb_1-xK_xCaF₃(x<or=0.2) mixed perovskite crystals

Abstract

The influence of ionic substitution on the phase transitions in mixed Rb1-xKxCaF3 fluoroperovskite crystals (x<or=0.2) is studied by EPR of the Gd3+-O2- pair substituted for a Ca2+-F- bond as a local probe. The behaviour of the EPR spectra versus temperature reveals the existence of two successive first-order phase transitions associated with CaF6 octahedron rotation. Computer reconstructions of the EPR spectra allow us to conclude that, in the low-temperature phase, the CaF6 octahedra are rotated by three equal angles around the fourfold cubic axes. Then, we were able to determine the variations of the total rotational angle phi defined by phi 2= phi (100)2+ phi (010)2+ phi (001)2=3 phi /sub /(001)2. It is observed to increase continuously for x=0.05 whereas a discontinuity towards larger values occurs in the x=0.2 compound. The phase diagram drawn from the EPR measurements shows that the transition temperatures are enhanced with increasing x and the temperature range over which the tetragonal phase exists tends to become smaller. These results lead to the conclusion that the occurrence of the phase transitions is related to the compactness of the unit cell and that the substitution enhances the instabilities at the M and R points of the Brillouin zone.

Structural phase transitions in KMnF3doped by Li+, Na+and Rb+

“…In crystals with Li+ the low-temperature phase is monoclinic due to the frozen tilting of octahedra around three pseudocubic axis type of a-b+a-. A resultant rotation is around [l 111 axis, similar as was observed in (Rb1-,K,)CaF3 (Debaud-Minorel et al, 1994).…”

Re-Interpretation of EPR Spin-Hamiltonian Parameters bm2 for Gd3+-O2- Center in the Tetragonal Phase of Perovskite-Type Compound Rb1-xKxCaF3 with x = 0.05

“…The structural phase transitions of substituted perovskite compounds, such as Rb 1±x K x CaF 3 , have recently been studied by means of several methods [1 to 3]. The phase transitions of Rb 1±x K x CaF 3 (x 0.2) were studied by the spectrum of electron paramagnetic resonance (EPR) around the Gd 3+ ±O 2± pair substituted in the crystal for a Ca±F bond as a localized structural probe [2,3].…”

“…The structural phase transitions of substituted perovskite compounds, such as Rb 1±x K x CaF 3 , have recently been studied by means of several methods [1 to 3]. The phase transitions of Rb 1±x K x CaF 3 (x 0.2) were studied by the spectrum of electron paramagnetic resonance (EPR) around the Gd 3+ ±O 2± pair substituted in the crystal for a Ca±F bond as a localized structural probe [2,3]. It is found that by the rotation of the (CaF 6 ) 4± octahedron, RbCaF 3 shows a weakly first-order phase transition from cubic to tetragonal symmetry at T c 195 K, while Rb 1±x K x CaF 3 two successive first-order ones [2,3].…”

“…1. Oxyz and OXYZ axis systems for the tetragonal phase (note: in cubic phase f 0, both systems are the same) T 165 K [3]. Considering that in the tetragonal phase the (GdOF 5 ) 4± octahedron is rotated around the``xº axis by an angle f and assuming that the rotated octahedron is rigid-like, Debaud-Minorel and Buzare [3] obtained the expressions of parameters b m 2 in the Oxyz axis system in terms of the spin-Hamiltonian parameters b 0 H 2 of this tetragonal Gd 3+ ±O 2± center in cubic phase (OXYZ axis system) by using reference frame transformation,…”

“…Substituting the rotational angle (or order parameter) f % 4.8 and b 0 H 2 À2590 Â 10 ±4 cm ±1 [3] into Eq. (1), they found that for the parameters b À1 2 and b 0 2 , the calculated values are in agreement with the observed values, however, for the parameter b 2 2 , the calculated value is different from the observed value (see Table 1).…”