Ruth E A McKnight

Acoustic dissipation associated with phase transitions in lawsonite, CaAl2Si2O7(OH)2·H2O

Abstract Resonant ultrasound spectra of a single crystal and a polycrystalline sample of lawsonite [CaAl2Si2O2(OH)2·H2O] have been measured at room temperature and at low temperatures in the region 20-300 K. The influence of known phase transitions at 125 and 270 K is seen in the frequency variations of the resonance peaks, which are indicative of elastic stiffening, and in values for the quality factor QQF, which are indicative of dissipation. Two dissipation peaks, at ~250 and ~210 K, are interpreted as being due to the proton order-disorder processes associated with the two species of hydrogen atoms in the structure: one in hydroxyl OH groups and one in the H2O molecules. These occur below the Cmcm ↔ Pmcn transition point but coincide with changes in the shear elastic constants and in features of IR spectra reported elsewhere. A third, much smaller, dissipation peak occurs immediately below the Pmcn ↔ P21cn transition point. The combination of these anomalies in acoustic dissipation and in elastic constants is consistent with the view that the Cmcm ↔ Pmcn transition is driven both by displacive and proton ordering effects. For the Pmcn ↔ P21cn transition, dissipation and the transition are more closely related, consistent with the view that the transition is driven essentially by proton ordering.

Elastic anomalies associated with transformation sequences in perovskites: II. The strontium zirconate-titanate Sr(Zr,Ti)O 3 solid solution series

The sequence of phase transitions due to octahedral tilting across the Sr(Zr,Ti)O(3) solid solution series has been investigated by resonant ultrasound spectroscopy at high and low temperatures using ceramic samples. The elastic behaviour associated with phase transitions as a function of composition in Sr(Zr,Ti)O(3) at room temperature is proposed to be analogous to that as a function of temperature in SrZrO(3), with the [Formula: see text] transition at SrZr(0.57)Ti(0.43)O(3), [Formula: see text] at SrZr(0.35)Ti(0.65)O(3), and [Formula: see text] at SrZr(0.05)Ti(0.95)O(3). Changes in elastic constants and acoustic dissipation with temperature have been analysed for samples across the compositional range. The intermediate phases, I4/mcm and what is assumed to be Imma, appear to have stability fields across the full compositional range and both show large dissipation effects, most probably due to twin wall mobility. In contrast, the Zr-rich Pnma phase, which should contain transformation twin walls, is an unexpectedly stiff and non-dissipating material, similar to the high temperature and/or Ti-rich [Formula: see text] phase. In the case of Pnma, this is attributed to coupling between the two order parameters, which could impede relaxation responses to an applied stress. The [Formula: see text] structure is a classically stiff cubic perovskite and no transformation-related dissipation processes are expected.

Elastic anomalies due to structural phase transitions in mechanoluminescent SrAl2O4:Eu

RUS facilities in Cambridge were established with the support of grant to M.A.C. from the Natural Environment Research Council of Great Britain, Grant No. NE/ B505738/1. C.J.H. and M.A.C. acknowledge the support of the Leverhulme Trust in the form of a Visiting Professorship for C.J.H. R.L.W. and Y.L. gratefully acknowledge the financial support from the Australian Research Council in the form of ARC Discovery Grants.

Elastic relaxations associated with the Pm\bar {3} m –R\bar {3} c transition in LaAlO3: IV. An incipient instability below room temperature

Resonant ultrasound spectroscopy has been used to characterize elastic softening and acoustic dissipation behaviour in single crystal and ceramic samples of LaAlO(3) between 10 and 300 K. For the twinned R3c single crystals, average values of the cubic elastic moduli (1/2)(C(11) - C(12)) and C(44) were followed while the ceramic sample provided data for the bulk and shear moduli. A Debye-like dissipation peak occurs in the vicinity of 250 K, from which an activation energy of 43 ± 6 kJ mol(-1) has been obtained. The mechanism for this is not known, but it is associated with C(44) and therefore could be related in some way to the cubic <--> rhombohedral transition at ∼817 K. Slight softening in the temperature interval ~220 --> 70 K of resonance peaks determined by shear elastic moduli hints at an incipient E(g) ferroelastic instability in LaAlO(3). The softening interval ends with a further dissipation peak at ∼60 K, the origin of which is discussed in terms of freezing of atomic motions of La and/or Al away from their high symmetry positions in the R3c structure. LaAlO(3) thus shows evidence of incipient structural instability at low temperatures which is potentially analogous with the phenomenologically rich behaviour of SrTiO(3).

Structural changes in zirconolite under α-decay

We report the simulation of alpha recoil cascades in zirconolite (CaZrTi(2)O(7)) using the molecular dynamics method under a variety of simulation conditions. Interatomic potentials have been developed through fitting to crystalline and melted structures and their energy surfaces obtained from ab initio calculations. Single and double cascades have been modelled, showing that damage recovery in damaged crystalline material is greatly impaired when there are multiple events. The calcium ions are significantly easier to displace than other ions, which is consistent with a recent ab initio study of defect energies.

The influence of grain structures on resonance behavior near phase transitions.

Inhomogeneities such as grains or plastically deformed regions in materials locally change sound wave velocities and introduce scatter in the pattern of expected resonance frequencies. The accompanying variation in the components of the free energy may promote, or block, nucleation and growth of new phases near transition temperatures. Lanthanides and martensites are two groups of materials with complex phase structures where control of the phase transformation is important in technological applications. Complex oxide minerals with a wide variety of phase structures are important in understanding the behavior of the earth’s crust. The influence on mechanical resonances of both the micro (meso) structures and the appearance and growth of phases near transition temperatures will be discussed. Experimental data on resonance behavior in inhomogeneous materials (largely lanthanides, transition metals, and silicates), where attempts have been made to control the scale and type of microstructure as they approach...