S.C. Abrahams

Ferroelectric lithium niobate. 3. Single crystal X-ray diffraction study at 24°C

Abstract A comprehensive X-ray study of lithium niobate at room temperature has been made, prompted by the recently discovered excellent nonlinear optical properties and unusual ferroelectric behavior of this crystal. Lithium niobate is rhombohedral with lattice constants aH = 5·14829 ± 0·00002, cH = 13·8631 ± 0·0004 A and space group R3c(C3v6), at 23°C. The cry structure has been solved without reference to earlier work, and refined using three-dimensional Patterson and Fourier series in addition to the method of least squares. The integrated intensities of 247 independent, 486 dispersively-unequal hkl and khl, reflections in all forms within a hemisphere of (sinθ) λ = 1·02 A−1 were measured with PEXRAD. The final agreement factor was 0.038. The absolute configuration of the atomic arrangement has been established, and for the first time in a ferroelectric crystal, related to the sense of the ferroelectric polarization, as determined by pyroelectric measurement followed by etching. The oxygen atoms are arranged in planar sheets, forming a network of distorted octahedra. At 24°C, the sequence of octahedra along the cH−axis is, Nb, vacancy, Li, Nb, vacancy, Li. The Nb octahedra have two Nb-O distances, of 1·889 ± 0·003 and 2·112 ± 0·004 A. The Li octahedra also have two metal-oxygen distances, Li-O being 2·068 ± 0·011 and 2·238 ± 0·023 A. There is metallic contact between Nb and Li, with puckered sheets of Nb and Li atoms 3.054 A apart normal to cH connected by 3.010 A contacts parallel to cH.The thermal vibrations of all atoms are isotropic and correspond to a characteristic Debye temperature of 503°K.

Ferroelectric lithium niobate. 5. Polycrystal X-ray diffraction study between 24° and 1200°C

Abstract The atomic arrangement of LiNbO 3 remains essentially unchanged from 24° to 1200°C, with the z -coordinate of oxygen ( z (O)) increasing from 0.0647 at 24°C to 0.0702 at 1200°C. It is proposed that at the Curie temperature (1210°C), z (O) becomes 1 12 and y (O) becomes 1 3 (from 0·3446 at 24°C). The niobium position then acquires the symmetry of an inversion center, and the space group becomes R 3 instead of R 3 c . Movement of the Li and Nb point charges, from nonpolar R 3 to ferroelectric R 3 c , is in agreement with the sign of the ferroelectric polarization experimentally determined. The thermal vibrations at 1210°C are large, and at 1253°C the crystal melts. The linear thermal expansion coefficients are α a = 16·7 × 10 −6 ° C −1 between 24° and 800°C, α c ≈ 2 × 10 −6 ° C −1 between 24° and 600°C; the volume expansion coefficient is β = 36·5 × 10 −6 ° C −1 between 24° and 1000°C.

Ferroelectric lithium tantalate—III. Temperature dependence of the structure in the ferroelectric phase and the para-electric structure at 940°K☆

Abstract The neutron scattering from a single crystal of LiTaO 3 (Curie temperature 907°K) has been measured at 760, 820, 885 and 940°K, and has been remeasured at 298°K. Least squares structural refinement of the five data sets shows that as the temperature approaches T c the oxygen atom approaches the position x, 1 3 , 1 12 , with respect to Ta at the origin, in space group R 3 c . The temperature variation of the oxygen y - and z -coordinates is very similar to that of the spontaneous polarization. The lithium atom position below T c remains essentially invariant as a function of temperature. At T c , the oxygen atom occupies the position x, 1 3 , 1 12 , the lithium atom becomes disordered and distributed over the positions 00z and 0, 0, 1 2 -z , and the tan alum atom becomes located at an inversion center, in space group R 3 c . The lithium atom sites above T c lie 0.374 A on either side of the oxygen atom plane at z = 1 4 .

The crystal structure and magnetic properties of the rare-earth nickel (RNi) compounds

Abstract The crystal structure of GdNi has been accurately determined and the lattice constants of eleven isomorphous rare-earth nickel (RNi) compounds, with the CrB type structure, measured. The magnetization of these compounds has been measured as a function of temperature in a field of 14.24 kOe. Saturation magnetization measurements were made at 4·2°K in fields up to 80 k0e. The effective paramagnetic moments agree excellently with the theoretical moments of free R 3+ ions. The Curie temperatures of the RNi compounds have been determined and compared with those of the RNi 2 and RNi 5 compounds. The spin moments appear ferromagnetically aligned parallel with the crystal b -axis, normal to the puckered sheets of R and Ni atoms comprising the crystal. A single crystal of SmNi exhibits a coercive force of 35 kOe at 1·4°K. The most important interatomic distances are Ni-Ni at 2·614, Gd-Ni at 2·903, 2·917 and 2·954 A, and Gd-Gd at 3·588 and 3·631 A. The smoothed volume change between LaNi and LuNi is −17 per cent.

Transition metal iodates. II. Crystallographic, magnetic, and nonlinear optic survey of the 3d iodates

The anhydrous iodates of Cr, Mn, Fe, Co and β-Ni form a single isomorphous family, crystallizing in space group P63 or P6322 with lattice constants typified by Mn(IO3)2 of a = 11.178 ± 0.002, c = 5.035 ± 0.001 A and four formulas per unit cell; in addition, α-Ni(IO3)2 forms as a second phase. Mn(IO3)2, Fe(IO3)3, α-Ni(IO3)2 and β-Ni(IO3)2 order antiferromagnetically at 6.5, 17.0, 3.5 and 5.0 K, respectively; Cr(IO3)3 and Co(IO3)2 remain paramagnetic to 1.5 K. Below ΘN, a weak ferromagnetic moment develops in the Mn, α-Ni and β-Ni iodates. All the anhydrous iodates generate second harmonics. Co(IO3)2 · 4H2O and β-Ni(IO3)2 · 4H2O crystallize isomorphously in space group P21c, with lattice constants a = 8.370 ± 0.005, b = 6.572 ± 0.007, c = 8.514 ± 0.008 A, β = 99.8 ± 0.1° for the Co compound. Co(IO3)2 · 2H2O is triclinic, with a = 6.666 ± 0.015, b = 10.991 ± 0.025, c = 4.913 ± 0.011 A, α = 93.1 ± 0.1, β = 92.1 ± 0.1, γ = 98.9 ± 0.1°, space group P1, and Ni(IO3)2 · 2H2O is orthorhombic, a = 9.14986 ± 0.00008, b = 12.20896 ± 0.00022, c = 6.58353 ± 0.00013A at 298 K, space group Pbca. The Co iodate hydrates are paramagnetic to 1.5 K; both Ni hydrates are antiferromagnetic, the dihydrate also developing a weak ferromagnetic moment. The lattice spacings of all 11 compounds are presented, 9 with indexing.

Transition metal iodates. VII. Crystallographic and nonlinear optic survey of the 4f-iodates

Thirteen crystallographically distinct families of 4f-iodates, including hydrates, have been investigated. The anhydrous Type I family, extending from Ce to Lu, crystallizes in the monoclinic system, space group P21a: The lattice constants of Gd(IO3)3, for example, are a = 13.389 ± 0.006, b = 8.500 ± 0.002, c = 7.106 ± 0.002 A, β = 99.73 ± 0.03° with four formulas per unit cell. Yb(IO3)3 and Lu(IO3)3 also crystallize in Type II with monoclinic space group P21c and lattice constants for Yb(IO3)3 of a = 8.685 ± 0.005, b = 6.066 ± 0.002, c = 16.687 ± 0.009 A, β = 115.01 ± 0.18°, and four formulas per unit cell. Polycrystalline samples only of the anhydrous Types III, IV, V, and VI have been prepared and typical powder patterns are given. All anhydrous 4f-iodates form in centrosymmetric space groups. La and Ce grow as hemihydrates in the orthorhombic space group C2221, with a = 19.26 ± 0.01, b = 7.40 ± 0.01, c = 6.76 ± 0.01 for La(IO3)3·12H2O, and both generate second harmonics more efficiently than quartz. Ce, Pr, Nd, Pm, and Sm form monohydrates, space group P21, with lattice constants for Sm(IO3)3·H2O of a = 10.080 ± 0.007, b = 6.642 ± 0.006, c = 7.250 ± 0.008 A, β = 112.9 ± 0.1°, and two formulas per unit cell. The monohydrates are also more efficient than quartz at generating second harmonics. Two dihydrated families grow: Type I from Tm to Lu and Type II from Nd to Er, both triclinic. Lu(IO3)3·2H2O has a = 8.018 ± 0.012, b = 9.956 ± 0.021, c = 6.969 ± 0.016 A, α = 99.8 ± 0.2°, β = 93.8 ± 0.2°, γ = 68.2 ± 0.2° with two formulas in the unit cell, space group P1. Nd(IO3)3·2H2O has a = 7.56 ± 0.04, b = 10.77 ± 0.05, c = 7.34 ± 0.02 A, α = 105.3 ± 0.4°, β = 110.8 ± 0.7°, γ = 97.9 ± 0.6° with two formulas per cell and space group P1. Polycrystalline Gd to Lu(IO3)3·4H2O, and Ce to Sm(IO3)3·5H2O Type I form in centrosymmetric space groups; powder patterns for two tetrahydrates and the four pentahydrates are given. La(IO3)3·5H2O and Pr(IO3)3·5H2O, Type II, are monoclinic, space group P21m, with lattice constants for Pr(IO3)3·5H2O of a = 6.768 ± 0.008, b = 23.120 ± 0.039, c = 7.107 ± 0.007 A, β = 112.7 ± 0.1°, and four formulas per unit cell.

Transition metal iodates. IV. Crystallographic, magnetic and nonlinear optic survey of the copper iodates

Abstract Three anhydrous polymorphs of cupric iodate, two hydrates, and the basic iodate salesite have been investigated. α-Cu(IO 3 ) 2 is monoclinic, space group P 2 1 , with a = 5.551 ± 0.008, b = 5.101 ± 0.004, c = 9.226 ± 0.010 A and β = 95°4′ ± 11′, with two formulas in the unit cell. Below Θ N = 8.5 K, α-Cu(IO 3 ) 2 is antiferromagnetic and also pyroelectric. β-Cu(IO 3 ) 2 is triclinic, space group P 1 , with a = 11.230 ± 0.006, b = 11.368 ± 0.009, c = 10.630 ± 0.009 A, α = 99°18.3′ ± 0.3′, β = 107°0.4′ ± 0.2′ and γ = 114°23.8′ ± 0.2′ and eight formulas per unit cell: the crystal is paramagnetic to 1.4K. γ-Cu(IO 3 ) 2 is monoclinic, space group P2 1 m , with a = 4.977 ± 0.004, b = 6.350 ± 0.004, c = 8.160 ± 0.004 A and β = 92°20′ ± 4′, with two formulas per unit cell; γ-Cu(IO 3 ) 2 becomes antiferromagnetic below Θ N = 5 K. Cu(IO 3 ) 2 ·2H 2 O is monoclinic, space group P2 1 c , with a = 6.725 ± 0.005, b = 4.770 ± 0.007, c = 11.131 ± 0.013 A and β = 103°1′ ± 4′, with two formulas per unit cell; Cu(IO 3 ) 2 ·2H 2 O is paramagnetic to 1.4 K. Cu(IO 3 ) 2 · 2 3 H 2 O (mineral bellingerite) is triclinic, space group P 1 , with a = 7.197 ± 0.005, b = 7.824 ± 0.004, c = 7.904 ± 0.004 A, α = 105°2′ ± 2′, β = 97°7′ ± 2′ and γ = 92°54′ ± 2′ with three formulas per unit cell; this crystal is paramagnetic to 1.4 K, with a moderate antiferromagnetic Cu-Cu interaction. Cu(OH)IO 3 (mineral salesite) is orthorhombic, with a = 10.772 ± 0.004, b = 6.702 ± 0.002 and c = 4.769 ± 0.002 A and four formulas per unit cell. The magnetic susceptibility indicates the possibility of antiferromagnetic ordering at 162 K; strong antiferromagnetic interactions give Θ p = −340 K. The only copper iodate studied that generates second harmonics is α-Cu(IO 3 ) 2 . Indexed powder patterns are given for all six compounds.

Absolute sense of pyroelectric p3 and piezoelectric d33 coefficients in 3La (IO3)3.HIO3.7H2O

Abstract The pyroelectric coefficient p3 in 3La(IO3)3.HIO3.7H2O has an average value 2.0×10-5 Cm-2 in the temperature range 152 to 240 K. The resistivity decreases from 1012 to 1010 ohm-cm between 258 and 338 K. At 298 K, the piezoelectric coefficient d33 ue5fb 19×10-12CN-1. Positive polarity is generated on (001) by increasing temperature or tensile stress. A displacement toward (001) by La3+ or H3O+ ions of 1×10-4 A per K or 106Nm-2, or rotation of the water molecule or iodate ion dipoles by about 5 arc minutes per K or 106Nm-2, produces the observed polarity.

Atomic displacements in pyroelectric and piezoelectric crystals

Atomic displacements produced in pyroelectric crystals by a temperature change of one degree, or in piezoelectric crystals by the application of a stress of 106 Nm−2, are estimated in both cases to be on the order of 10−4 A. Corresponding polarization changes caused by dipole rotations are estimated to be on the order of minutes of arc. A group of 12 pyroelectric and 10 piezoelectric crystals are considered in which known absolute senses of pyroelectric or piezoelectric coefficients lead to a derivation of the sense of the ionic displacements.

Crystal chirality and optical rotation sense in isomorphous NaClO3 and NaBrO3

Abstract Combined optical and anomalous x-ray diffraction measurements confirm an earlier report that crystals of isomorphous NaClO 3 and NaBrO 3 with identical chirality (handedness of atomic arrangement) rotate plane polarized light in opposite senses.