S. Matyjasik

Pyroelectric properties of high-resistant KTiOPO4 crystals in the temperature range 4.2–300 K

The results of measuring the pyroelectric coefficient γsσ of nominally perfect KTiOPO4 (KTP) crystals grown from solution in a melt with a potassium to phosphorus ratio of ∼2 are presented. The γsσ (T) dependence is monotonic in the range from 4.2 to 250 K. Deviations from a linear dependence are observed beginning from 250 K, which is considered to be due to interstitial-potassium transport in the KTP crystal field. The spontaneous polarization of unclamped KTP samples is estimated from the results of the measurements. In terms of the crystal-physics approach, it is shown that the main contribution to a polar state of KTP is made by the dipole moments of two nonequivalent mesoscopic tetrahedra forming two sublattices that are polarized in opposite directions and bound by Ti(1) ions.

Pyroelectric properties of bismuth ferrite in the low-temperature range

The pyroelectric effect in bismuth ferrite single crystals is studied in the temperature range from 4.2 to 200 K (limited from above only by the high conductivity of the sample). A nonmonotonous character of the temperature dependence of the pyroelectric coefficient is revealed in the range 120–160 K: at T = 140 K, the pyroelectric coefficient reaches the maximum value equal to 1.2 × 10−4 C/(m2 K). The crystallophysical analysis of the BiFeO3 structure based on the structural data confirms that the octupole moment of the structure makes the main contribution to the spontaneous polarization. This circumstance leads to a small (in comparison with isostructural compounds) spontaneous polarization of bismuth ferrite and anomalous behavior of the pyroelectric coefficient caused by the contribution of local vibrations of strongly distorted TiO6 octahedra.

Pyroelectric properties of AlN wide-gap semiconductor in the temperature range of 4.2–300 K

The results of measurements of AlN polarization in the range of 4.3–300 K are presented. These results are used for calculating the pyroelectric coefficients as a function of temperature. The experiment was carried out both with an original sample grown from gas phase in nitrogen atmosphere at T ∼ 2400 K and with the sample subjected to external electric action in a field of different polarity. The sample polarization at T = 4.2 K results in a radical modification of the polar state of real AlN sample: due to the structural defects, ferroelectric ordering arises and the total sample polarization dependent on T increases by an order of magnitude. First of all, the revealed anomalies are related to the occurrence of oxygen in the crystalline structure resulting in considerable deformations of coordination tetrahedra in the wurtzite structure. The process of replacement of the nitrogen vacancies by oppositely charged oxygen ions also leads to changing the dipolemoment orientation for the coordination tetrahedra in the structure instead of changing only their magnitudes. The ferroelectric ordering “in pure form” exists in this AlN sample only below T ≈ 80 K.

Spontaneous polarization and pyroelectric effect in the improper ferroelectrics-ferroelastics Gd2(MoO4)3 and Tb2(MoO4)3 at low temperatures

The experimental variations in the spontaneous polarization ΔPs(T) and pyroelectric coefficient γs(T) for Gd2(MoO4)3 (GMO) and Tb2(MoO4)3 (TMO) at low temperatures reported here differ from those for intrinsic ferroelectrics. A fundamental difference is found in the repolarization behavior of samples of GMO and TMO at fixed temperatures of 300 and 4.2 K. While the single domain formation temperature essentially has no effect on the measurements for TMO, a fundamental difference is observed in the case of GMO: single domain formation in the latter at 4.2 K leads to an order of magnitude increase in ΔPs at T > 85 K and distinct anomalies are observed in γs(T), at one of which the pyroelectric coefficient reaches a record peak of 3 × 10−4 C/(m2·K) at T = 25 K. At T = 200 K the pyroelectric coefficients equal −1.45 and −1.8 in units of 10−6 C/(m2·K). Based on these results and taking published data on the rotational structural transformation in the (001) plane and symmetry considerations into account, we prop...

Pyroelectric properties of real LiNbO3 single crystals grown from a congruent melt

The temperature dependences of the pyroelectric coefficient of lithium niobate single crystals grown from a congruent melt have been investigated in the range of 4.2–300 K. No anomalies were found at low temperatures, and the experimental dependence is described well by the Debye-Einstein model, with TD = 357 K and two pyroactive frequencies of 692 and 869 cm−1. Specific features of lithium niobate have been analyzed. Two sublattices, formed by two pairs of mesotetrahedra with (according to the symmetry conditions) dipole and octupole moments, were selected in the structure. Their contributions to the total polarization differ by an order of magnitude. Vacuum annealing of the samples leads to the occurrence of anomalies only at temperatures over 280 K; these anomalities are interpreted as a manifestation of superionic conductivity.

Pyroelectric properties of lithium triborate in the temperature range from 4.2 to 300 K

Temperature dependences of pyroelectric coefficients γT and γS corresponding to the mechanically free (T) and clamped (S) states of a lithium triborate (LBO) sample are studied and found to be nonmonotonic. It is proposed that the anomalies are associated with an increase in anharmonism of the lithium sublattice of LBO long before its transition to the superionic state. The spontaneous polarization at T=200 K is estimated to be 0.25 C/m2. In the structural motif, the mesotetrahedra responsible for the emergence of spontaneous polarization of LBO are singled out.

Pyroelectric properties of the wide-gap semiconductor CdS in the low-temperature region

Spontaneous polarization, thermally stimulated conductivity and depolarization are comprehensively studied in the range from 4.2 to 300 K on nonstoichiometric n-type CdS crystals grown from the gas phase in an argon atmosphere at T = 1450 K. The objects of study are initial samples and samples polarized by a weak electric field at T = 4.2 K. Sample polarization results in a decrease in the conductivity σ33 due to restructurization of the entire energy level spectrum associated with the formation of donor-acceptor pairs. The latter processes also contribute to the temperature dependences of the spontaneous polarization and the pyroelectric effect, characterized by the formation of anomalies below 15 K and the formation of thermoelectret. The role of an uncontrollable oxygen impurity in the formation of CdS cationic conductivity above 270 K, associated with the decay of a fraction of donor-acceptor pairs, is discussed. In the temperature range from 20 to 250 K, the pyroelectric coefficient and spontaneous polarization are independent of external influences within experimental error; at T = 200 K, they are ΔPs = −(6.1 ± 0.2) × 10−4 C/m2 and γs = −(4.1 ± 0.3) × 10−5 C/m2 K.

Characteristic of spontaneous polarization in Li2Ge7O15 crystals

The temperature dependence of spontaneous polarization in Li2Ge7O15, a ferroelectric with one polarization axis, is studied in the temperature range from 4.2 K to TC. An unusual character of spontaneous polarization associated with the change of the crystal behavior from the Ising to multipole type is revealed in the vicinity of TC. The low value of spontaneous polarization is explained not by the relation between certain physical quantities, but rather by the interaction of the components of higher order (n > 2) electric moments which, by definition, have rather low values. The Li2Ge7O15 structure is built by macrofragments consisting of two types of mesotetrahedra whose distortion results in the appearance of pseudopolarization of the nonvector type.

Pyroelectric properties of potassium and rubidium acid phtalates

Abstract Results on measurements of temperature dependences of pyroelectric coefficients for potassium and rubidium acid phtalates are presented for temperatures ranging from 4.2 to 350 K. Based on the achieved results, the Debye and Einstein, temperatures for those crystals were estimated along with the wave number of phonons contributing to the pyroelectric effect.

Pyroelectric Properties of Potassium and Rubidium Titanyl-Arsenate Single Crystals in the Temperature Range of 4.2-300 K

The temperature dependences of the pyroelectric coefficients of KTiOAsO4 and RbTiOAsO4 single crystals grown by flux crystallization have been investigated in the temperature range of 4.2–300 K. With an increase in temperature, superionic conductivity first arises in KTiOAsO4 (at T > 200 K) and then (at T > 270 K) in RbTiOAsO4. This conductivity is much higher in the samples polarized at T = 4.2 K. An exponential change in the crystal resistivity along the polar direction is simultaneously observed. The results of measurements in the range of 4.2–200 K indicate larger values of pyroelectric coefficients when compared with potassium and rubidium titanyl-phosphate crystals. A correlation between the pyroelectric coefficients and a change in the lattice constants at isomorphic substitutions of K atoms for Rb and P atoms for As has been revealed within the symmetry approach.