College of Letters & Science
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Item Search for tricritical point in KH2PO4 at high pressure. I. Static dielectric behavior near critical point at zero pressure(1977-01) Western, Arthur B.; Baker, A. G.; Pollina, R. J.; Schmidt, V. HugoThe proposed tricritical point in KDP occurs if the critical field, Ecr, can be brought to zero by applying pressure. The Landau equation of state E = A0(T - T0)P + BP3 + CP5 gives straight-line “isopols” in the T-E plane. We obtain values for A0, B and C and thus Ecr by observing such isopols. We find A0 = 4.3 × 10-3; B = -2.35 × 10-11 C = 5.91 × 10-19 cgs esu for the crystal studied at ambient pressure. These values lead to Ecr = 232 V/cm and δPspon (Tc) = 1.82 C/cm2. High pressure results are imminent.Item Tricritical point and tricritical exponent δ in KH2PO4(1978-01) Schmidt, V. Hugo; Western, Arthur B.; Baker, A. G.; Bacon, Charles R.Static dielectric results for a KH2PO4 crystal at pressures of 0, 2, and 2. 4 kbar are analyzed in terms of a Landau free energy expansion using the “isopol” technique. The measured exponent δ at 2. 4 kbar is consistent with the mean-field tricritical value of 5. This result and the Landau parameter values indicate a tricritical point near 2. 4 kbar.Item Tricritical Point in KH_{2} PO_{4}.(1976-11) Schmidt, V. Hugo; Western, Arthur B.; Baker, A. G.Item Pressure-induced tricritical point in the ferroelectric phase transition of KH_{2}PO_{4}(1978-06) Western, Arthur B.; Baker, A. G.; Bacon, Charles R.; Schmidt, V. HugoMeasurement of the net polarization charge of two KH2PO4 crystals as a function of temperature, applied electric field, and hydrostatic pressure indicates the existence of a tricritical point in the (2.3±0.3)-kbar pressure range. This result is based upon static measurements of the polarization response to applied dc field in a 0.5-K neighborhood of the ferroelectric transition at pressures of 0, 1, 2, 2.4, and 3 kbar. Unlike He3- He4 mixtures and some metamagnets, for KH2PO4 the field which gives rise to the "wing" structure of the tricritical point is experimentally available. For each of the five pressures the paraelectric region is well described by the Landau equation of state, E=A0(T−T0)P+BP3+CP5, to within 0.05 K of the transition temperature. The exponent δ derived from our data at 3 kbar is in the crossover region between the critical value of 3 and the tricritical value of 5 predicted by Landau theory. At 2.4 kbar our derived value of δ is consistent with the tricritical value of 5. Analysis of the data along lines of constant polarization, which are here called "isopols," indicates that the transition is first order at 0 and 1 kbar with the critical field decreasing from 183±60 V/cm at 0 kbar to 43±13 V/cm at 1 kbar. At 3 kbar the B coefficient is positive which indicates a second-order transition. This observation of a change in the order of the transition is supported by a change in the behavior of the isothermal dielectric susceptibility which has a maximum for E>0 at 0.5 kbar and at E=0 at 3 kbar.Item Simple coulometer for studying protonic conduction in crystals(1965-12) Schmidt, V. HugoNOTES ON EXPERIMENTAL TECHNIQUE AND APPARATUS A simple glass coulometer with mercury electrodes was designed to measure the amount of gas evolved upon passing current through single crystals, to determine the extent to which electrical conduction in these crystals is due to protons or other gas-forming ions. It is useful if at least one coulomb can be passed through the crystal in a reasonable time interval. With single crystals of KH2PO4 and Li(N2H5)SO4 in this coulometer, the ratio of hydrogen evolution rate to crystal current indicates entirely protonic conduction.Item Dynamical Effects of Deuteron Intrabond Jumping in KD2PO4(American Physical Society, 1967-12) Schmidt, V. HugoDeuteron jumps along hydrogen bonds in KD2PO4 associated with effective motion of Slater-Takagi DPO4 and D3PO4 groups are related to the deuteron magnetic-resonance spectrum and relaxation both above and below the ferroelectric transition temperature Tc, and to the dielectric relaxation and the domain-wall mobility. Below Tc the deuterons assume ordered bond positions, but occasionally jump to the opposite positions. The correlation time for the electric-field-gradient fluctuations seen by a deuteron undergoing such motion is found to be short above Tc and even shorter below Tc. The rms amplitude of the fluctuations drops rapidly below Tc. These results are used to explain the temperature dependence of the splitting and lack of line broadening in the deuteron spectrum below Tc, and the rapid decrease of the spin-lattice transition probabilities below Tc. Relaxation data above Tc, which yielded the energies and fractional populations of DPO4 and D3PO4 groups, are reanalyzed using the recently reported field-gradient tensors at the two bond positions. The dielectric relaxation is calculated, and good agreement with the high dielectric loss measured at microwave frequencies is found. The jump time for a DPO4 or D3PO4 group is found to be near hkT, implying little correlation between successive jumps, in contrast to KH2PO4. This jump time is used in a calculation of domain-wall mobility in KD2PO4.Item Deuteron Intrabond Motion and Ferroelectricity in KD2PO4(American Physical Society, 1964-01) Silsbee, Henry; Uehling, Edwin A.; Schmidt, V. HugoThe Slater theory of the ferroelectric phase transition in KH2PO4 type crystals as modified by Takagi and extended by Senko is examined again in the light of recent measurements of parameters made on KD2PO4. Making full use of the measurements, and assigning an appropriate numerical value to one adjustable parameter which is not independently measured, the theory gives a nearly correct description of the shape of the spontaneous polarization curve. Also the value obtained for the Curie constant is probably satisfactory in view of limitations of the model and uncertainty of the high-temperature experimental data. The theory fails, however, in at least one important respect; the transition entropy is overestimated. This and other discrepancies are discussed briefly.