Phase Transitions in the Brominated Ferroelectric Tris‐Sarcosine Calcium Chloride
paraelectrics SrTiO 3 and KTaO 3 . [ 9 ] However, the ultra-weak nature of the dipole moment in TSCC referred to below may reduce the effects of such long-range dipole interactions to a narrow temperature region around the critical point. This would mean that more conventional classical or quantum critical behaviour would be expected over a wider range of temperatures and tuning parameters. TSCC is hydrogen bonded and was characterized in the past as order-disorder albeit with no direct supporting evidence. [ 16,17 ] Since the time of these studies TSCC has come to be a prototypical displacive ferroelectrics, [ 18 ] as demonstrated by the existence of an under-damped soft mode. This mode can be followed into the GHz frequency regime [ 19 ] from highT values of ca 630 GHz = 21 cm −1 . Deuteration [ 20 ] produces little change in T C , implying that the transition is not controlled by the N-Cl-hydrogen bonds, a result compatible with the soft-mode mechanism operating in TSCC. [ 2 ]
Quantum critical points in ferroelectric relaxors: Stuffed tungsten bronze 
K3Li2Ta5O15
 and lead pyrochlore (
Pb2Nb2O7
)
We have synthesized ceramic specimens of the tetragonal tungsten bronze K3Li2Ta5O15 (KLT) and characterized its phase transition via x-ray diffraction, dielectric permittivity, resonant ultrasonic spectroscopy, and heat capacity measurements. The space group of KLT is reported as both P4/mbm and Cmmm with the orthorhombic distortion occurring when there are higher partial pressures of volatile K and Li used inside the closed crucibles for the solid state synthesis. The data show strong relaxor behavior, with the temperature at which the two dielectric relative permittivity peaks decreasing, with 104≥Tm1≥69K and 69≥Tm2≥46K as probe frequency f is reduced from 1 MHz to 316 Hz. F tests show that the data satisfies a Vogel-Fulcher model better than Arrhenius with an extrapolated freezing temperature for ɛ′ and ɛ′′ of Tf1=+15.8 and −11.8K and Tf2=−5.0 and −15.0K for f→0 (tending to dc). This difference between Tf from real and imaginary values, albeit counterintuitive, is mandatory, according to the theory of Tagantsev. Therefore, by tuning frequency, the transition could be shifted to absolute zero, suggesting KLT has a relaxor-type quantum critical point. In addition, we have reanalyzed the conflicting literature for Pb2Nb2O7 pyrochlore which suggests that this also has a relaxor-type quantum critical point since the freezing temperature from the Vogel-Fulcher fitting is below absolute zero. Since the transition temperature evidenced in the dielectric data at approximately 100 kHz shifts below 0 K for very low frequencies, this transition would not be seen with heat capacity data collected in the zero-frequency (dc) limit. Both of these materials show promise for possible new relaxor-type quantum critical points with nonperovskite based structures.
Molecular Orbital Study of Proton Transfer in (H3NHOH2)
Ab initio molecular orbital methods are used to study transfer of the central proton in the asymmetric (H/sub 3/NHOH/sub 2/)/sup +/ system. Calculations are performed at the Hartree-Fock level with basis sets of split-valence (4-31G) and double-zeta plus polarization function (DZP) quality. The effects of electron correlation upon the transfer potentials are computed via the generalized valence bond (GVB) and polarization configuration interaction (POL-CI) techniques. The barrier to proton transfer between NH/sub 3/ and OH/sub 2/ is observed to heighten as the distance between the latter two molecules is increased. At the equilibrium R(NO) hydrogen-bond length, the transfer potential contains a single minimum, (H/sub 3/NH...OH/sub 2/)/sup +/, in which the central proton is more closely associated with NH/sub 3/. In both the rapid and adiabatic models of proton transfer, there is no energy barrier to decay of (H/sub 3/N...HOH/sub 2/)/sup +/ to the equilibrium (H/sub 3/NH...OH/sub 2/)/sup +/ structure. While all the calculations agree on the above points, there are some notable quantitative discrepancies between the various methods. Enlargement of the basis set at the Hartree-Fock level results in higher transfer barriers while subsequent inclusion of electron correlation (POL-CI) leads to barrier reductions. The GVB procedure, with its partial treatmentmore » of correlation, produces changes in the potentials opposite to those observed for the more complete POL-CI treatment.« less