A. Waśkowska

Physical properties and X-ray diffraction of a NiMn2O4 single crystal below and above the ferrimagnetic transition at Tc = 145 K

Abstract The magnetic and calorimetric measurements of stoichiometric NiMn 2 O 4 showed a phase transition at T c = 145 K. The single crystal X-ray diffraction investigations pointed to the purely magnetic nature of the transition. The magnetostriction effect, observed in the temperature dependence of the unit cell volume and persisting above T c , suggested a short-range spin ordering. The cation distribution over the tetrahedral and octahedral sites in the spinel structure has been determined; the inversion parameter v = 0.87. The role of Mn cations in the magnetic coupling process has been discussed. Our results allow the formula to be written with the Mn cations in two oxidation states on the octahedral site: (Ni 1 − v 2+ Mn v 2+ ) [Ni v 2+ Mn 2 − 2 v 3+ Mn v 4+ ] O 4

Spectroscopy and crystal structure of neodymium coordination compound with glycine: Nd2(Gly)6·(ClO4)6·9H2O

Abstract Neodymium complex compound with glycine: Nd2(Gly)6·(ClO46·9H2O was synthesized and obtained in the form of monocrystals. Absorption spectra recorded in the region of 8000–35 000 cm-1 were measured along the crystallographic axes. Intensities of the f-f transitions were analysed on the basis of the Judd theory. The X-ray crystal structure determination of the complex is reported. Crystals are triclinic, space group P I , with a = 11.554(4) A, b = 14.108(1) A, c = 15.660(3) A, α = 97.14(1)°, β = 102.82(2)°, γ = 105.28(1)°, V = 2355.25 A3 Z = 2, M.W. = 1495.4, Dc = 2.129)(3) g cm-3, Dm = 2.103(1) g cm-3. The structure was solved by Pattersons method and successive Fourier syntheses giving the locations of all nonhydrogen atoms. The final R factor was 0.062 and Rw = 0.073 for 12869 reflections with |Fo| > 5σ|(Fo)|. The asymmetric unit consists of a dimeric formula unit. The coordination polyhedron of Nd atoms comprises seven oxygen atoms from glycine and two from water molecules. The neodymium-glycine bonding mode is compared with that of the calcium-glycine complex.

Ferroelectric phase transition in hydrogen-bonded 2-aminopyridine phosphate (NC4H4NH2)·H3PO4

A new crystal of 2-aminopyridine phosphate (NC4H4NH2)H3PO4 has been grown and its x-ray structure and physical properties were studied. At room temperature the crystals are monoclinic, space group C2/c. The flat 2-aminopyridine cations are hydrogen bonded to the anionic [PO4 ] groups. The interesting feature of the crystal structure is the three-dimensional network of hydrogen bonds including, among others, two strong, symmetrical O H, H O interactions with disordered proton locations. Symmetrically related PO4 anions linked through these protons form infinite (PO4)∞ chains along the crystal a-axis. The anomalies in the temperature dependence of the electric permittivity showed that the crystal undergoes ferroelectric phase transition at Tc = 103.5 K. The spontaneous polarization takes place along the crystal a-axis, being parallel to the chains of the hydrogen-bonded PO4. The disordered protons, thermally activated at room temperature, can be frozen at their positions in the ferroelectric phase. The order–disorder continuous type of the transition has been evidenced on the basis of the temperature dependences of electric permittivity and spontaneous polarization measurements.

Spectroscopy and crystal structure of holmium and dysprosium complex compounds with glycine: Ln(Gly)3(H2O)3(ClO4)3

Abstract Lanthanide(III) complexes with the formula Ln(C2H5NO2)3(H2O)3(ClO4)3 (where Ln ≡ Ho, Dy) were obtained in the form of monocrystals which were isomorphic and crystallized in monoclinic space group Cc with the following cell constants: Ho(C2H5NO2)3(H2O)3(C1O4)3: a = 20.506(3) b = 9.245(1) c = 23.989(4) A β = 100.28(1)° V = 4474.7(2) A 3 Z = 8dc = 2.20 g cm−3dm = 2.19(2)MR = 742.53 F(0.00) = 2912 Dy(C2H5NO2)3(H2O)3(ClO4)3: a = 20.56(7) b = 9.42(8) c = 24.16(5) A β = 98.7(5)°Z = 8 dm = 2.19 Results from the X-ray crystal structure determination are given for the Ho3+ complex compound. The coordination polyhedron of a Ho(III) ion comprises seven oxygen atoms from glycine and two from water molecules. Two oxygen bridges fasten the linear polymer running along the b axis. Absorption spectra recorded in the region 5500–40000 cm−1 were measured along the a axis for the Dy3+ complex and the probabilities of f-f transitions were analysed on the basis of the Judd-Ofelt theory. Solid state fluorescence spectra of the Dy3+ ion were recorded at 77 and 300 K. The results are discussed and the Stark levels have been determined. Spectroscopic properties of all the known Dy3+ carboxylates were compared.

Structural phase transitions in tetra(isopropylammonium) decachlorotricadmate(II), [(CH3)2CHNH3]4Cd3Cl10, crystal with a two‐dimensional cadmium(II) halide network

Single crystals of tetra(isopropylammonium) decachlorotricadmate(II) as a rare example of a two-dimensional cadmium(II) halide network of [Cd(3)Cl(10)](n)(4-) have been synthesized and characterized by means of calorimetry and X-ray diffraction. The crystals exhibit polymorphism in a relatively narrow temperature range (three phase transitions at 353, 294 and 259 K). Our main focus was to establish the mechanism of these successive transformations. The crystal structure was solved and refined in the space group Cmce at 375 K (Phase I), Pbca at 320 K (Phase II) and P2(1)2(1)2(1) (Phase III) at 275 K in the same unit-cell metric. The structure is composed of face-sharing polyanionic [Cd(3)Cl(10)](4-) units which are interconnected at the bridging Cl atom into four-membered rings forming a unique two-dimensional network of [Cd(3)Cl(10)](n)(4-). The interstitial voids within the network are large enough to accommodate isopropylammonium cations and permit thermally activated rotations. While in Phase I isopropylammonium tetrahedra rotate almost freely about the C-N bond, the low-temperature phases are the playground of competition between the thermally activated disorder of isopropylammonium cations and stabilizing N-H···Cl hydrogen-bond interactions. The transition from Phase I to II is dominated by a displacive mechanism that leads to significant rearrangement of the polyanionic units. Cation order-disorder phenomena become prominent at lower temperatures.

Temperature dependent lattice instability in single crystals of ferromagnetic CdCr2Se4 diluted with In and Sb

In ferromagnetic CdCr2Se4 diluted with Mex3+ = In and Sb, deviations from cubic symmetry appear in the paramagnetic phase just below room temperature, and they increase with decreasing temperature. For Sb admixture, the unit-cell anomalies indicate a structural phase transition to occur at the same temperature as the magnetic transition, Tc = 130?K, which also is the same Tc as for the parent crystal CdCr2Se4. The low temperature phase has been described in orthorhombic space group Fddd. For In admixture, a structural transition occurs in the paramagnetic state at about Ta?200?K (which is higher than Tc = 125?K), to a tetragonal structure with space group I41/amd. This behaviour is attributed to macroscopic spontaneous strain due to chemical heterogeneities, and to spin frustrations due to mixed valencies of Cr. The paramagnetic Curie?Weiss temperature ?C?W decreases for both admixtures, indicating changes in competing ferromagnetic and antiferromagnetic interactions. The magnetization at 2.1?K exhibits saturation for H>0.6?T. The magnetic moments are ?sat = 6.26?and 5.47??B?mol?1 for Sb and In admixtures, respectively. These values are consistent with the Cr3+ and Cr2+ mixed valencies, and with the proposed cation distribution model. A spin?phonon coupling of the transitions in the crystal with Sb admixture is suggested, while such a correspondence is not clear for the In admixture.

Structure and phase transition in pyridazine perchlorate

Single crystals of pyridazine perchlorate [(C4H4N2)HClO4] have been synthesized and characterized by x-ray diffraction, dielectric measurements and optical studies. At room temperature the crystal is monoclinic, space group P 21/n. The temperature dependences of the electric permittivity and birefringence exhibit anomalous behaviour, characteristic of the first-order phase transitions at 343 and 339 K on heating and cooling, respectively. Optical investigations under a polarizing microscope show that the crystal symmetry changes from the biaxial room temperature phase II to the optically uniaxial high temperature phase I. The domain pattern arising after cooling from phase I reflects observed symmetry changes at phase transition. The fully disordered structure of phase I can be described in the obverse hexagonal space group .

The crystal structure of triglycine fluoberylate in ferroelectric and paraelectric phase

The crystal structure of triglycine fluoberylate has been refined in the ferroelectric phase at a temperature of 23°C and in the paraelectric phase at 97°C using the data collected on a diffractometer CAD-4. Positions of all hydrogen atoms in both phases have been determined. The refinement of paraelectric phase in a space group without symmetry centre gives a good result in overcoming the difficulties resulting from the overlapping of atoms.

A high-pressure x-ray diffraction study of a phase transition in KMnF3

Abstract Unit-cell parameters of KMnF 3 single crystals have been measured by X-ray diffraction under hydrostatic pressure up to 9.5 GPa in a diamond-anvil cell. A cubic/tetragonal structural phase transition has been observed at P c = 3.1 GPa . The tetragonal phase was stable to the upper limit of the measured pressure range ( P = 10.0 GPa ) and a linear pressure dependence of the macroscopic spontaneous strain ϵ s above P c has been shown. The linear and volume compressibilities of both phases have been calculated.

X-ray study of phase transitions of NH4BeF3

Abstract Lattice parameters of NH4BeF3 have been measured in the temperature range 235–365 K with an X-ray single-crystal Bond diffract meter. DSC measurements and Weissenberg photographs are also described in both the high-and low-temperature regions, and the phase diagram is presented. On heating, NH4BeF3 transforms at 334 K to an intermediate phase and at 347 K to the high-temperature phase.