Magnetostimulated changes of microhardness in potassium acid phthalate crystals

Abstract

1 The infuence of weak magnetic felds on the mechanical properties of nonmagnetic crystals (magnetoplastic effect) is observed in both micro-[1] and macroplasticity [2][3][4][5][6][7][8][9] and is studied by scientists who use plastic physics methods.The studies started in 1987 [1] showed that the magnetoplastic effect is explained by the action of a magnetic feld on spin-dependent electron transitions either in a system dislocation-paramagnetic center [10,11] or in magnetosensitive complexes of point defects [12,13]. The electron transitions stimulated by magnetic felds may change the local energy of the dislocation interaction with a point defect, which results in plasticization (or hardening [14]) of a number of diamagnetic alkali halide, semiconductor, and metal crystals [10,11]. As was shown earlier, similar processes may also change the rates of chemical reactions in magnetic felds [15,16]. Up to now, the study of the magnetoplastic effect has been performed mainly on isotropic crystals. Interest has arisen in the study of the infuence of a magnetic feld on the properties of nonmagnetic anisotropic crystals with complicated structures widely used in technology.We studied a potassium acid phthalate (KAP) crystal of the composition C 8 H 5 O 4 K with ionic, covalent, and hydrogen bonds [17,18]. An orthorhombic crystal is described by the point symmetry group mm2. The crystal properties are explained by the presence of the polar 001 axis and a large distance between the (010) cleavages (~13 Å). Figure 1 shows the structural formula of KAP crystals possessing piezoelectric properties [19] that are widely used as analyzers in the longwavelength range of the X-ray spectrum and as monochromators in various high-resolution X-ray instruments [20]. In the temperature range from 300 to 2 K, KAP crystals are diamagnetic along all the three main crystallographic directions. SAMPLE PREPARATION AND PROCESSING OF RESULTSThe experimental methods used in our study were selected in accordance with the studies of mechanical properties of KAP crystals [21][22][23] and, in particular, of their microhardness [23]. Motion of dislocations was not studied in any cited works. In [21,22], considerable anisotropy of crystal deformation was observed: depending on the mutual orientation of the compression and polar (z) axes, crystals demonstrated brittle fracture, kink bands, or plastic deformation. To detect the infuence of a magnetic feld on the crystals we used a fast technically simple method of microindentation.The KAP crystals used in our experiments were grown from aqueous solution by the method of decreasing temperature [19] at the Institute of Crystallography of the Russian Academy of Sciences. The samples were cut from crystalline boules by a wet thread. The faces were mechanically polished against a smooth wet silk. Microhardness was measured in the (010) and (100) 2 Magnetic susceptibility of KAP crystals was measured by Yu.G. Shvedenkov at the International Tomography Center, Siberian Division, Russian Academy of Sciences, ...

Change in the microhardness of nonmagnetic crystals after their exposure to the Earth’s magnetic field and AC pump field in the EPR scheme

“…The reversibility of the observed magnetic effects was reached after a much longer pause (of the order of 1-2 weeks). The same delay of the microhardness reversibility after treatment of ZnO and KAP crystals in a static mag B netic field was previously observed in [21][22][23]. The delay of the recovery of the crystal microhardness sen sitivity to magnetic exposures was detected for the first time in C 60 crystals [25].…”

“…When the crystal is exposed to the dc magnetic field, the effect is well reproducible for all samples cut from one growth pyramid of the same crystal [22,23]. In this case, in the B || x geometry, the effect was com pletely suppressed [23].…”

“…The choice of these objects is dictated by the fact that memory effects were previously detected in them upon exposure to a dc magnetic field. In ZnO and KAP, the memory effects were detected by changes in their microhardness [21][22][23]; in TGS, this was done by the transformation of its ferroelectric properties [24].…”

Perspective on Terahertz Applications of Molecular Crystals

“…The nickel sulfate hexahydrate NiSO 4 •6H 2 O (NSH) crystal that was used as a UV radiation filter has a similar microhardness: H NSH ∼760 MPa [201]. For the semiorganic nonlinear crystal of potassium hydrogen phthalate KC 8 H 5 O 4 (KAP), it is H KAP(010) ∼1600 MPa [202]. The widely known inorganic nonlinear optical crystal KH 2 PO 4 (KDP) has a microhardness of H KDP ∼2250 MPa [203].…”

Magnetostimulated softening and hardening of semiconductors

“…Later the influence of the weak magnetic field (µB<<kT, where µ -Bohr magneton, B -magnetic induction, k -Boltzmann constant, T -temperature) on mechanical properties of crystals such as yield stress [2], microhardness [3], coefficient of strengthening [4], internal friction [5,6] was turned out. This effect was observed in alkali-halide crystals [1][2][3][4][5][6][7][8][9][10], non-magnetic metals [7], polymers [11,12], molecular crystals [13], semiorganic crystals [14] and ferroelectrics [15,16]. Complex researches of MPE have allowed to attribute it to the range of spin-dependent phenomena determining magnetosensitive interactions between lattice defects.…”