Yu. V. Shaldin

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.

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.

Magnetic investigations of Cd1−xZnxTe (x=0.12, 0.21) wide-gap semiconductors

The results of measuring the magnetization M and magnetic susceptibility χ of Cd1−xZnxTe crystals are presented. The hysteresis of the M(H) dependence, which is caused by the presence of arbitrarily oriented magnetic clusters, is observed in magnetic fields H<2 kOe. Van Vlek paramagnetism, which is caused by electric fields of defects, makes a substantial contribution to the magnetic susceptibility. The anomalies in the χ(T) dependence in the temperature region T<50 K are associated with variation in the charge state of interstitial Te for x=0.12. For x=0.21, these anomalies can be caused either by the paramagnetism of noninteracting defects or by antiferromagnetic ordering of the defect subsystem formed by ZnCd and Tei. The effect of annealing on the magnetic state of a defect subsystem in the samples is ascertained.

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.

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

In the temperature range T = 4.2–300 K, the spontaneous polarization of nonstoichiometric CdSe samples grown by sublimation at 1423 K and subjected to annealing in selenium vapors is measured. From these data, the temperature dependences of the pyroelectric coefficient in CdSe are calculated. Anomalies are found in the ranges T < 10 K and T > 210 K. A change in the charge state of the defect subsystem of the samples caused by a weak electric field results in strengthening of the anomalies and in the occurrence of a new anomaly at T = 236 K. As a rule, the anomalies at T < 10 K and T = 236 K depend on the polarity of the external action. The action of shunting by the intrinsic conductivity of the sample manifests itself only in the region above 270 K. Investigations of the nonpolar-cut sample leads to somewhat unexpected results: the anomalies are retained, but their values considerably decrease. Within the framework of the crystallophysical approach, attempts are made to explain the anomalies observed below 270 K related both to associates with a dipole moment and to their transformation with the possible participation of an uncontrollable oxygen impurity in macroformations of the cluster type, the orientation of the total dipole moment of which does not coincide with the polar direction of an ideal crystal.

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.

Magnetic Properties of High-Resistivity CdTe〈In〉 and CdTe〈Cl〉 Crystals

The magnetic susceptibility of high-resistivity CdTe〈In〉 and CdTe〈Cl〉 crystals was measured between 4.2 and 300 K. The susceptibility was found to vary anomalously with temperature. Below 50 K, all the samples were paramagnetic. The observed anomalies are interpreted in terms of donor–acceptor pairs formed by native defects and dopant or uncontrolled impurity atoms. The effect of doping on the 300-K χ is related to the Van Vleck paramagnetic contribution resulting from the local electric fields of Xi–VCdand Ini–VCddefect complexes. In CdTe〈Cl〉, this contribution is insignificant.