NH4H2PO4 Crystals Research Articles

An analysis of flow and mass transfer of solution growth of NH4H2PO4 crystals by the modified seed mounting geometries

A modified seed mounting geometry is proposed, aiming to improve the distribution homogeneity of the supersaturation on the crystal face during the growth of ammonium dihydrogen phosphate (NH4H2PO4, ADP) crystals. To evaluate the modification, three dimension transient numerical simulation of flow and solute transfer involved in the growth of ADP crystals have been performed. The effects of flow on the surface supersaturation distribution is analyzed in detail. Compared with the ordinary seed mounting geometry, the modified geometry is able to suppress the formation of flow stagnation regions near the prismatic surface and improve the magnitude and the distribution homogeneity of the supersaturation. Particularly even at a lower rotation rate (ω = 30 rpm), the distribution of the surface supersaturation still possesses a better uniformity under the modified geometry. Meanwhile, with the increase of rotation rate, there is an improvement in the magnitude and the distribution of the supersaturation on the crystal surfaces. The effects of natural convection adjacent to prismatic faces can be ignored when rotation rate is equal to or greater than 50 rpm. Moreover, as the crystal size increases, the magnitude of the supersaturation on the prismatic surface declines a little, and its uniformity deteriorates gradually. Consequently, as the crystal growing, it should increase the rotation rate appropriately. Finally, etching studies reveal that the ADP crystals with higher quality can be obtained by modifying the seed mounting geometry simply.

Electronic Structure and Optical Property Calculation of an Oxygen Vacancy in NH4H2PO4 Crystals**Supported by the National Natural Science Foundation of China under Grant No 51402173, the Fundamental Research Funds for the Central Universities under Grant No FRF-TP-15-099A1, and the Funding Project of China Scholarship Council under Grant No 201806465071.

The electronic structure of perfect ammonium dihydrogen phosphate (ADP) and defective ADP with an oxygen (O) vacancy are calculated by screened-exchange hybrid density functional HSE06. The optimized structural parameters of the defective ADP crystal are analyzed. The PO4 tetrahedron with an O vacancy is distorted and its symmetry is broken. The band gap of the defective ADP with an O vacancy is about 1.5 eV lower than the perfect ADP, which is due to the new O vacancy defect states near the valence band maximum. Moreover, more peaks appear in the low-energy region (lower than 6 eV) in the curves of the linear optical properties for the defective ADP. The results indicate that the O vacancy will significantly influence the laser damage performance of ADP crystals.

Effects of deuteration on the structure of NH4H2PO4 crystals characterized by neutron diffraction**Project supported by the National Natural Science Foundation of China (Grant No. 51402173), Shandong Provincial Natural Science Joint Foundation with Universities and Scientific Research Institution, China (Grant No. ZR2017LEM006), the Neutron Physics Laboratory, China Academy of Engineering Physics (Grant No. 2014BB07), and the Fundamental Research Funds for Central

A series of deuterated ammonium dihydrogen phosphate (DADP) crystals were grown and their structures were investigated by using powder neutron diffraction method. In the entire composition range, the deuterated level in the crystals is lower compared with the aqueous growth solution. The deuterium segregation coefficient in the crystals decreases with increasing deuterium content of the solution. The deuterium content in the group is higher than that in group. In addition, the variations of lattice parameters are shown here.

Anisotropy of acousto-optic figure of merit in tetragonal crystals with accounting for non-orthogonality of acoustic eigenwave polarizations. 1. The cases of KH2PO4 and NH4H2PO4 crystals

Anisotropy of acousto-optic figure of merit in tetragonal crystals with accounting for non-orthogonality of acoustic eigenwave polarizations. 1. The cases of KH2PO4 and NH4H2PO4 crystals

Dielectric, electromechanical, and elastic properties of Rb1– x (NH4) x H2PO4 single crystals

The longitudinal dielectric, piezoelectric, and elastic characteristics of ferroelectric RbH2PO4 and antiferroelectric NH4H2PO4 crystals are calculated in terms of the modified model of proton ordering with allowance for the piezoelectric coupling in an approximation of a four-particle cluster. The results obtained agree well with known experimental data. Along with this, the electromechanical properties of a Rb0.2(NH4)0.8H2PO4 single crystal undergoing an antiferroelectric phase transition have been studied experimentally over a wide temperature range. The model predictions and the experimental data agree qualitatively.

Kinetics of electron tunneling transfer in KH2PO4 and NH4H2PO4 crystals with hydrogen bonds

A model of electron transfer by tunneling between trapped electron and hole centers in crystals with hydrogen bonds under the conditions of thermostimulated mobility of one carrier type in the recombination process has been developed. The proposed model describes all features in the kinetics of induced optical density relaxation observed in nonlinear optical crystals of KH2PO4 (KDP) and NH4H2PO4 (ADP) on a wide temporal scale (10−8–10 s) under pulsed irradiation. The results of model calculations have been compared with experimental data on the photoinduced transient optical absorption (TOA) in KDP and ADP crystals in the visible and UV ranges. The nature of the radiation-induced defects, which account for the TOA, and the dependence of the TOA decay kinetics on the temperature, excitation power, and other experimental conditions have been considered.

Features of the behavior of longitudinal static and dynamic dielectric, piezoelectric, and elastic characteristics of KH2PO4 and NH4H2PO4 crystals

Along with their electromechanical coupling coefficients, the longitudinal dielectric, piezoelectric and elastic characteristics in ferroelectric KH2PO4 and antiferroelectric NH4H2PO4 crystals are calculated using a modified proton ordering model that considers piezoelectric coupling and the four-particle cluster approximation. The possibility of detecting piezoactivity in solid solutions of K1 − x(NH4)xH2PO4 is substantiated.

Ionic and electronic processes in non‐linear optical crystals

The paper presents the results of a study of the formation and decay of lattice defects in nonlinear optical crystals of NH4H2PO4 (ADP), KH2PO4 (KDP), Li2B4O7 (LTB) and LiB3O5 (LBO) with a sublattice of mobile hydrogen (ADP, KDP) and lithium (LTB, LBO) cations. By means of the luminescent and absorption optical spectroscopy with (the) a nanosecond time resolution under excitation with an electron beam, it was revealed that the optical absorption of these crystals in the visible and UV spectral ranges is produced by optical hole-transitions from the local defect level to the valence band states. The valence band density of the states determines the optical absorption spectral profile, and the relaxation kinetics is rated by the interdefect radiationless tunnel recombination between the trapped hole center and the H0 and Li0 electron trapped centers. At 290 K, the H0 and Li0 centers are subject to thermally stimulated migration. All manifestations of a radiative recombination observed in these crystals are accounted for by the involvement of additional electronic and hole centers of a different nature in the recombination process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature time-resolved vacuum UV spectroscopy of NH4H2PO4 crystals

A complex investigation of the dynamics of electronic excitations in nonlinear optical crystals of ammonium dihydrophosphate NH4H2PO4 was performed using low-temperature vacuum UV luminescence spectroscopy with time resolution upon selective photoexcitation by synchrotron radiation. Data on the photoluminescence decay kinetics, time-resolved photoluminescence spectra (2–6.2 eV), and time-resolved photoluminescence excitation spectra (4–24 eV) were obtained for the first time for NH4H2PO4 crystals at 8 K. It is ascertained that the photoluminescence of NH4H2PO4 crystals in the vicinity of 4.7 eV has intrinsic character due to the radiative annihilation of self-trapped excitons. Possible channels of generation and decay of relaxed and unrelaxed electronic excitations in NH4H2PO4 crystals are discussed.

New seignette-electrics

Abstract Single crystals of KH2PO4, KH2AsO4, NH4H2PO4 and NH4H2AsO4 were grown from aqueous solutions. The dielectric constants parallel and perpendicular to the tetragonal axis of the crystals were measured by an ac-method as a function temperature. The spontaneous polarizations were determined from hysteresis loops at saturation. Curie temperatures of 115 K and 91 K for KH2PO4 and KH2AsO4 respectively were found from a Curie-Weiss plot. The ammonium salts do not show ferroelectricity. The crystals of NH4H2PO4 and NH4H2AsO4 disintegrate into small pieces at 155 K and 220 K approximately, due to a structural transition apparently. Attention is drawn to the existence of O—H…O bonds in these crystals which might be important for the occurrence of seignettc-electricity.

EPR studies of x-ray irradiated KH2AsO4 and KH2PO4-type ferroelectrics and antiferroelectrics

EPR has been used to characterize the hyperfine couplings and electronic structure of a new paramagnetic center (H2AsO4)2− in x-irradiated ferroelectric crystals of pure KH2AsO4 and mixed KH2PO4–KH2AsO4, and antiferroelectric mixed crystals of NH4H2PO4–NH4H2AsO4, NH4H2PO4–KH2AsO4, and ND4D2PO4–KD2AsO4. This center is formed by the capture of an electron by proton deficient AsO3−4 units in these crystals. The assignment of the center is supported by EPR studies on the temperature dependence of the proton superhyperfine structure of these crystals. The present study also corrects an earlier identification of this center in NH4H2PO4–KH2AsO4 as a hole-type ionic defect center AsO2−4. Our results also provide a direct experimental basis for the currently accepted model of electrical (protonic) conduction in these crystals and help to resolve an earlier controversy regarding conductivity measurements on sulfate-doped ammonium and potassium salts. In conjunction with x-ray photoelectron spectroscopic studies, the EPR results on doped crystals also provide interesting results on electron-capture processes in single crystals. The present work also provides an explanation of some anomalous trends of the hyperfine coupling variation in other XO4-type radicals.

Über das Wachstumsverhalten Kleiner Impfkristalle von Ammoniumdihydrogenphosphat in einer modifizierten Zirkulationsapparatur

AbstractWe have investigated the crystal growth of small seed crystals of NH4H2PO4. The crystal growth was measured by a modified circulation apparatus. The measured values are compared with those by other investigators and the reasons of differences in crystallization discussed. The influence of cobalt nitrate in solution on crystallization velocity and the impurity in crystals was investigated. A new crystal holder in the apparatus is described.