Phase Transition In RbCaF3 Research Articles

A high-pressure neutron diffraction study of the ferroelastic phase transition in RbCaF3

The fluoroperovskite phase RbCaF3 has been investigated using high-pressure neutron powder diffraction in the pressure range ~0–7.9 GPa at room temperature. It has been found to undergo a first-order high-pressure structural phase transition at ~2.8 GPa from the cubic aristotype phase to a hettotype phase in the tetragonal space group I4/mcm. This transition, which also occurs at ~200 K at ambient pressure, is characterised by a linear phase boundary and a Clapeyron slope of 2.96 × 10−5 GPa K−1, which is in excellent agreement with earlier, low-pressure EPR investigations. The bulk modulus of the high-pressure phase (49.1 GPa) is very close to that determined for the low-pressure phase (50.0 GPa), and both are comparable with those determined for the aristotype phases of CsCdF3, TlCdF3, RbCdF3, and KCaF3. The evolution of the order parameter with pressure is consistent with recent modifications to Landau theory and, in conjunction with polynomial approximations to the pressure dependence of the lattice parameters, permits the pressure variation of the bond lengths and angles to be predicted. On entering the high-pressure phase, the Rb–F bond lengths decrease from their extrapolated values based on a third-order Birch–Murnaghan fit to the aristotype equation of state. By contrast, the Ca–F bond lengths behave atypically by exhibiting an increase from their extrapolated magnitudes, resulting in the volume and the effective bulk modulus of the CaF6 octahedron being larger than the cubic phase. The bulk moduli for the two component polyhedra in the tetragonal phase are comparable with those determined for the constituent binary fluorides, RbF and CaF2.

Influence of the cationic substitution on the mechanismof structural phase transitions in RbCaF3 and KCaF3: study of a typical mixed crystal Rb0.68K0.32CaF3

Influence of the cationic substitution on the mechanismof structural phase transitions in RbCaF3 and KCaF3: study of a typical mixed crystal Rb0.68K0.32CaF3

Hydrostatic pressure electron paramagnetic resonance studies of the phase transition discontinuity in RbCaF3 up to 800 MPa

The influence of hydrostatic pressure on the cubic-to-tetragonal phase transition in RbCaF3 is studied by monitoring the EPR spectra of the Gd3+-O2- pairs and Gd3+ centres. The phase transition remains of first order up to 800 MPa and the discontinuity of the order parameter phi at Tc slightly increases with increasing pressure. Similarly, the nonlinearity of the cubic parameter b40(T), caused by the rotational fluctuations, near Tc becomes larger under pressure. The pressure shift of the phase transition temperature is determined: dTc/dp=(36.5+or-0.3) K GPa-1. The values of Tc as well as dTc/dp are interpreted within the rigid-sphere model.

Photoluminescence of RbCaF3:Mn2+: the influence of phase transitions

Precise photoluminescence measurements on an RbCaF3:Mn2+ sample containing only 400 p.p.m. of Mn2+ have been carried out in the 10-300 K temperature range. The results are compared with those obtained in other fluoroperovskites doped with Mn2+. The analysis of the 6A1g(S) to 4T1g(G) excitation peak at room temperature leads to a Mn2+-F- distance R=213.3 pm which is close to that derived from the experimental isotropic superhyperfine constant, As. The plot of the first moment of the emission band, M1, against temperature reveals a slight but sensible change of slope at T=193 K which is associated with the Oh1 to D4h18 structural phase transition of the host lattice. Furthermore, at T=40 K, M1 undergoes an abrupt increase of approximately 100 cm-1. This fact supports the existence of another phase transition involving an increase Delta R/R approximately=0.2% upon cooling, and thus a situation which is similar to that detected in the structural phase transition of KMnF3 at Tc3=81.5 K. To the authors' knowledge this is the first time that clear evidence of both phase transitions in RbCaF3 has been achieved through an optical probe. Finally the variation of the 4A1g(G), 4Eg(G) peak, E3, along the fluoroperovskite series is analysed.

Critical behaviour of [110]-stressed RbCaF3

Abstract We have investigated the cubic to tetragonal phase transition in RbCaF3 under [110] uniaxial stress by monitoring the EPR lines of the cubic Gd3+ centre which shift proportionaly to the square of the order parameter (the rotational anngle of the CaF6 octahedra Φ) or those of the Gd3+-O2- pair which splits linearly in Φ.

The structural phase transition in RbCaF3: an87Rb nuclear magnetic resonance investigation at various pressures

The authors have studied the cubic-to-tetragonal structural phase transition in RbCaF3 at atmospheric and high pressures by nuclear magnetic resonance techniques. The spin-lattice relaxation time T1 and the quadrupole coupling constant vQ have been measured near the phase transition at 13350 and 5400 bar, and thus the pressure dependence of the transition temperature Tc has been deduced. They compare their results with those concerning other perovskites. A two-dimensional Fourier-transform method based on the quadrupole interaction has been used to show phase coexistence at P=1 bar. At high pressure the authors have developed a method based on pulse-length excitation to detect any phase coexistence. Finally they discuss the change of the phase transition with increasing pressure.

Critical fluctuations in RbCaF3. I. A high-resolution X-ray scattering study

High-resolution X-ray scattering measurements have shown that the critical fluctuations at the cubic to tetragonal (196 K) structural phase transition in RbCaF3 exist on two length scales. The shorter length scale (some tens to hundreds of angstroms) corresponds to the soft-phonon mode of R25 symmetry and does not diverge at Tc. The longer length scale (some thousands of angstroms) is associated with the presence of defects and diverges at Tc. Experimental studies of the length scale of the critical fluctuations are complicated by the domain structure of the tetragonal phase. A general, cubic-phase Bragg reflection can split into a cluster of up to 15 tetragonal-phase Bragg reflectors. The arrangement of this cluster of Bragg reflections in reciprocal space and the corresponding domain structure in the sample crystal are discussed in detail.

Phase Transitions in RbCaF3 and KCaF3

Calculations have been performed to determine the lowest static energy configurations associated with R-pint lattice instabilities in RbCaF3 and KCaF3. Minimum static energies and ionic displacements have been obtained for rotations of the Fluorine octahedra about the [1, 0, 0] [1, 1, 0] and [1, 1, 1] directions. Lattice configurations and energy minima will be discussed and compared with experimental values.

Dynamics and disorder in RbCaF3above and below Tc=195K

The first-order Oh1 to D4h18 phase transition in RbCaF3 (Tc=195K) is investigated through detailed studies of EPR lines of Gd3+-O2- substituted for a Ca2+-F- bond. Above and below Tc the results are interpreted in the framework of Rousseau's model of octahedra precession. Above Tc, they give evidence for the two-dimensional character of the octahedra correlations, previously invoked to explain the fluctuation-induced first-order transition. The results are fully consistent with the authors' previous work and substantiate dynamic disorder and precursor order above Tc, in agreement with recent renormalisation group theories.

Phenomenological description of the first order improper ferroelastic phase transition in RbCaF3at 194 K.

Abstract Many studies have been devoted to investigate the cubic to tetragonal phase transition in RbCaF3 at 194 K. In order to describe the weak first order character of this phase transition, we have carried out calculations with a phenomenological Hamiltonian which contains, in addition to the expression proposed by SLONCZEWSKI and THOMAS, the six order terms of the order parameter. The results, obtained for the temperature dependence of the lattice parameters, the specific heat and the elastic constants, give a good qualitative description of the anomalies observed in the vicinity of the phase transition.

Nuclear magnetic resonance studies of the structural phase transitions in RbCaF3

Fourier Transform NMR measurements made on a single RbCaF3 crystal using Rb indicate a structural phase transition at 198 K. The temperature dependence of the quadrupole coupling constant and hence of the order parameter was studied in the range 130 K – 290 K by measuring the second order shift of the central line of Rb resonance. The critical exponent, β, of the order parameter at the transition is found to be equal to 0.33.

The cubic to tetragonal phase transition in RbCaF3 studied with neutron diffraction

The cubic to tetragonal phase transition in RbCaF3 studied with neutron diffraction

Investigation of the critical scattering at the structural phase transition in RbCaF3 using Mössbauer diffraction

Abstract The elastic and inelastic scattering of 14.4 keV Mossbauer gamma-rays from the 1/2(311) reflection of RbCaF3 was investigated in the critical region. The temperature dependence of both intensities near the phase transition at 193 K is in good agreement with other experiments.

The 193 K phase transition in RbCaF3

We report some new crystallographic and Raman scattering data on the 193 K phase transition in RbCaF3. Below Tc, the temperature dependence of the order parameter deduced from the relative distortion (c - a)/a is well described by a critical exponent β = 0.33.

Phase transitions in RbCaF3. I: Optical studies

Abstract Phase transitions in RbCaF3 have been investigated by optical birefringence measurements and Raman scattering experiments. A near discontinuity in the onset of spontaneous birefringence at 196 K shows that the cubic to tetragonal phase change is “slightly” first order. The Raman spectra of the tetragonal phase support a D184h structure in which two phonons of A1g and Eg symmetry soften as the 196 K transition is approached from lower temperatures. A very slow transition to a lower symmetry structure was observed at about 42 K in the Raman measurements.

Phase transitions in RbCaF3

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