The 193 K phase transition in RbCaF3

A new model for the phase transitions due to octahedra rotations in the perovskite structure

“…To get an order of magnitude of ~ da and dy, we consider that in the absence of lower temperature phase transition, the quadratic structure is entirely ordered at 0 K. Then, assuming that a(T) follows the same law as the pseudo-cubic lattice parameter 3 aq cq (Fig. 5), we can easily evaluate ~ doc(r), dy(T) from the lattice parameters aq and cq deduced from X-ray measurements in SrTi03 [18], RbCdF3 [2] and RbCaF3 [19]. The calculated values reported in figure 6 indicate that dy(T) follows a Landau type behaviour of the form ~(7~ -T) 112 the lack of volume discontinuity at the transition implies that da + dp + dy = 0 near 7~.…”

The temperature dependence of the structures of RbCaF3, and the determination of the low temperature phases

“…Transitions such as those discussed above are Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198000410108700 encountered in RbCaF3, in particular the 193 K (1) transition (Oh to D4h18) [13,14]. Moreover the additional presence of a central peak in the cubic phase [15] increases the interest of the Debye-Waller factor and tilt angle determination in this compound.…”

“…[16], two phase transitions occur at -50 K and -7 K. A previous study [14] showed that this line is split ( fig. 7b) From this one can see that the cell parameters 2a, 2b, 2c are nearly equal together with a great tendancy of equality of the tilts.…”

The 193 k phase transition in RbCaf3 : I lattice dynamics

“…Therefore we have focused our attention on the fluoperovskites of general formula AMF3 [3] and specially ACaF3 where A is a monovalent ion. In this sequence of compounds, CsCaF3 is still cubic at 4 K [4], RbCaF3 undergoes a transition from a cubic (Oh) to a tetragonal (D") space group at 193 K [5][6][7][8][9] and KCaF3 is already distorted from the cubic symmetry at room temperature [10]. Some raw data were published previously and show unambiguously the coexistence of a central peak and a soft phonon near the 193 K phase transition in RbCaF3 [11].…”

“…The modes are labelled according to a choice of origin at the Rb atom. The Raman frequencies are taken from reference [7] ; the data measured at Saclay and Grenoble are from the second part of this work. The fitting was carried out for the three transverse infrared-active optical modes and for the lower zone boundary modes ( 10 meV) of the X, M and R points.…”