College of Letters & Science

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Author: search.filters.author.Baker, A. G.End date: 1979

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    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. Hugo
    The 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.
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    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.
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    Tricritical Point in KH_{2} PO_{4}.
    (1976-11) Schmidt, V. Hugo; Western, Arthur B.; Baker, A. G.
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    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. Hugo
    Measurement 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.
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