Monoclinic Site Research Articles

Stability of monoclinic phase in pure and Gd-doped HfO2: a hyperfine interaction study

The present Time Differential Perturbed γ-γ Angular Correlation (TDPAC) measurements in bulk HfO2 report, for the first time, the presence of three regular sites at ambient condition, apart from the stable monoclinic phase (m-HfO2). Also, the effects of Gd impurity (5 at%) on HfO2 crystal structure was investigated by PAC. In pure HfO2, apart from the monoclinic component (~76%), four other minor frequency components have been found at room temperature. To further investigate the origin of these minor frequency components, PAC measurement at 673 K was performed. At 673 K, population of two of the four minor components was enhanced at the expense of the m-HfO2 as the population of m-HfO2 reduces to ~58%. On doping with 5 at% Gd, the population of monoclinic site drastically reduced to ~10% while one of the minor components got enhanced to a large extent (68%). The enhancements of the three minor components, both by temperature or by Gd impurity, indicate that these are the regular phases of HfO2 and could therefore be attributed to three orthorhombic phases of HfO2. The fourth minor component (5–10%) with values of ωQ ~ 7 Mrad/s, η = 0 can be assigned to crystalline defect site.

Temperature dependence of local structural changes around transition metal centers Cr3+ and Mn2+ in RAl3(BO3)4 crystals studied by EMR

Theoretical investigations are carried out to determine the temperature dependence of the local structural parameters of Cr3+ and Mn2+ ions doped into RAl3(BO3)4 (RAB, R = Y, Eu, Tm) crystals. The zero-field splitting (ZFS) parameters (ZFSPs) obtained from the spin Hamiltonian (SH) analysis of EMR (EPR) spectra serve for fine-tuning the theoretically predicted ZFSPs obtained using the semi-empirical superposition model (SPM). The SPM analysis enables to determine the local structure changes around Cr3+ and Mn2+ centers in RAB crystals and explain the observed temperature dependence of the ZFSPs. The local monoclinic C2 site symmetry of all Al sites in YAB necessitates consideration of one non-zero monoclinic ZFSP (in the Stevens notation, b21) for Cr3+ ions. However, the experimental second-rank ZFSPs (D=b20, E=1/3b22) were expressed in a nominal principal axis system. To provide additional insight into low symmetry aspects, the distortions (ligand's distances ΔRi and angular distortions Δθi) have been varied while preserving monoclinic site symmetry, in such way as to obtain the calculated values (D, E) close to the experimental ones, while keeping b21 close to zero. This procedure yields good matching of the calculated ZFSPs and the experimental ones, and enables determination of the corresponding local distortions. The present results may be useful in future studies aimed at technological applications of the Huntite-type borates with the formula RM3(BO3)4. The model parameters determined here may be utilized for ZFSP calculations for Cr3+ and Mn2+ ions at octahedral sites in single-molecule magnets and single-chain magnets.

The aging micromechanisms of alumina matrix composite (AMC) used in total hip arthroplasty.

The aim of the present study is to simulate and better understand the long-term interplay between alumina matrix composite (AMC) femoral head and aqueous environment. In particular, we focused on clarifying the role of alumina grains on zirconia phase stability and mechanical equilibrium in AMC head during aging processes in a clinically-relevant time frame. The tested AMC head consists of an alumina matrix (82 vol.%) reinforced by yttria-stabilized zirconia (17 vol.%), chromium oxide (0.5 vol.%), and strontium aluminate (0.5 vol.%). The accelerated hydrothermal aging testing (at 134°C, 2-bars pressure) was conducted up to 20h. According to ASTM standard, 1-h aging under these conditions corresponds approximately to 2yrs invivo exposure. Confocal Raman and fluorescence spectroscopy were applied to quantify surface tetragonal-to-monoclinic phase transformation of zirconia and surface stresses in the AMC head. The Mehl-Avrami-Johnson (MAJ) theory was applied, which allowed modeling the micromechanisms of nucleation and growth of monoclinic sites during the transformation process. The obtained results were compared to those of monolithic zirconia (3Y-TZP) femoral heads. The 3Y-TZP head showed a rapid increase of transformation rate beyond the aging time of 5h (simulated as ∼10yrs invivo), suggesting the initiation of the transformation cascade toward the neighboring zirconia grains (growth mechanism). On the other hand, MAJ analysis revealed that the growth mechanism was completely absent and the nucleation of the monoclinic phase was partially prevented in the AMC head even after the 20-h aging (∼40yrs invivo). In addition, the stress accumulation in the AMC head was restricted at a quite low level throughout the aging simulation. Those results suggest that the presence of stable and hard alumina in the AMC can play a considerable role in slowing down the destabilization processes by spatially encompassing zirconia grains.

Reconciling in vivo and in vitro kinetics of the polymorphic transformation in zirconia-toughened alumina for hip joints: II. Theory

In this paper, an updated analytical model for the kinetics of the tetragonal to monoclinic (t→m) polymorphic transformation in ZTA composites for hip joints is proposed and discussed. The model builds upon the so-called Mehl-Avrami-Johnson (MAJ) description of transformation kinetics, which combines two overlapping processes: nucleation of monoclinic sites, and their successive growth. Dependencies on two specific factors are introduced in the model, namely the initial fraction of monoclinic polymorph as received from the manufacturer, and the presence of different types of transition-metal stains (e.g., Ti, CoCr, and Fe) on the ZTA surface. These two factors were studied because clear indications of their potential roles on the environmental stability of implantable ZTA components were found in previous phenomenological analyses of retrievals. Nucleation and growth of monoclinic domains are two key processes whose interplay decides the actual kinetics of the overall transformation process according to two main parameters: an apparent activation energy value function of time and temperature, and a nuclei growth exponent. These parameters were clearly altered by the presence of transition metal contamination, whose effect was incorporated into the model to explain exacerbations of surface degradation. In accordance with a general analytical description of transformation kinetics for isothermal or isochronal evolutions in terms of time and temperature, the modified model of the MAJ description assesses the effect of the initial monoclinic fraction. The updated model has been validated using systematic in vitro experiments, and appears to partly reconcile in vitro and in vivo data of t→m transformation discrepancies in ZTA hip components.

Standardization of crystal field parameters for rare-earth (RE3+) ions at monoclinic sites in selected laser crystals

Survey of spectroscopic studies of trivalent rare-earth (RE3+) ions at orthorhombic or monoclinic sites in various crystals, which are important in view of promising technological applications, has revealed a number of crystal field (CF) parameter (CFP) sets belonging to different ranges in the CF parameter space. This indicates that the major properties of orthorhombic and monoclinic CF Hamiltonians have not been fully realized in literature. This includes the existence of alternative, i.e. physically equivalent, CF parameter (CFP) sets and incorrectness of comparison of such alternative yet disparate CFP sets, which due to their intrinsic features cannot be directly compared. Proper methodology for dealing with such CFP sets has been outlined earlier. In this paper we consider implications of these properties for interpretation of (i) spectroscopic data for rare-earth ions obtained from several experimental techniques and (ii) the axis systems in which the fitted CFPs are supposedly expressed. Since these aspects are not realized by many researchers, incorrect conclusions concerning, e.g. the strength of CF and structural implications, have often been drawn. The standardization approach is employed for meaningful comparative analysis of the CFP sets for RE3+ ions at monoclinic sites in selected laser crystals. The following ion-host systems are studied: Nd3+: LuAlO3, Nd3+: BaY2F8, Eu3+: Gd2O3, Eu3+, Pr3+, Tb3+, Ho3+, Tm3+, Nd3+, Sm3+, Dy3+, Er3+: Y2O3, Er3+: Er2O3, Pr3+, Sm3+, Gd3+, Er3+: LaF3, Er3+: YAlO3. A representative sample of CFP sets is analyzed, standardized, and presented in a unified way. The alternative CFP sets provided here may be utilized in the multiple correlated fitting technique to improve reliability of final fitted CFPs. The present approach may facilitate future spectroscopic determination of CFPs as well as increase reliability of fitted and theoretical CFPs for fN (RE3+) ions at lower symmetry sites.

Comparative analysis of crystal-field parameters for rare-earth ions at monoclinic sites in AB(WO4)2 crystals: II. Pr3+ and Nd3+ ions in KRE(WO4)2 (RE = Y or Gd), Pr3+ ions in M+Bi(XO4)2 (M+ = Li or Na and X = W or Mo), and Nd3+ ions in NaBi(WO4)2 and AgNd(WO4)2

In part I, the crystal-field (CF) parameter (CFP) sets for important potential solid state laser systems Tm3+, Ho3+, and Er3+ ions in KGd(WO4)2 and Tm3+ ions in KLu(WO4)2 were thoroughly revisited using a general framework for the analysis of CF levels and CFP modeling. In this part the non-standard CFP sets for Pr3+ and Nd3+ ions in KR(WO4)2 (R = Y or Gd) and the standard CFP sets for Pr3+ ions in M+Bi(XO4)2 (M+ = Li or Na and X = W or Mo) and Nd3+ ions in the related systems NaBi(WO4)2 and AgNd(WO4)2 are analyzed. Due to structural similarity of the hosts, the CFP values for a given trivalent rare-earth (RE3+) ion should be quite close in these systems. However, the fitted (and model) CFP sets appear disparate for the systems in question. The standardization criteria are utilized to ensure direct comparability of the apparently disparate CFP sets reported in the literature. The CFP sets standardized by us are compared with the originally standard CFP sets for Pr3+ and Nd3+ ions in related AB(XO4)2 systems. Following part I, we argue that meaningful analysis of the mixed CFP sets, i.e. standard and non-standard ones, must take into account the intrinsic features of CF Hamiltonians for orthorhombic and lower symmetry cases, which have not been fully recognized in the literature as yet. The model or fitted CFP sets that belong to disparate regions in the CFP space are intrinsically incompatible, i.e. such sets should not be directly compared. The correlated alternative CFP sets are calculated using monoclinic standardization transformations. The closeness of the standardized CFP sets is assessed in a quantitative way using the closeness factors and the norms ratios. Comparative analysis of the monoclinic CFP sets reported for the titled ion–host systems is carried out and several inconsistencies in the previous studies are clarified. The CFP sets determined by standardization are utilized as starting sets for applications of the multiple correlated fitting technique to independently obtain and additionally verify the fitted CFPs based on published energy levels data. Multiple correlated fittings offer an advantage over the single-fitting tactics by enabling an improved fine-tuning of the final fitted CFPs as well as their interpretation and comparability with the sets obtained by others. The present consistent methodology may enable better understanding of the intricate aspects inherent in the spectroscopic studies for other ion–host systems exhibiting orthorhombic, monoclinic, and triclinic site symmetry.

Structural, down- and phase selective up-conversion emission properties of mixed valent Pr doped into oxides with tetravalent cations

We report on structure-property relationships in Pr-doped CeO2 and ZrO2 using X-ray diffraction (XRD), Raman, UV to Vis Diffuse Reflectance (DR-UV/Vis), X-ray Photoelectron (XPS), and luminescence (PL) spectroscopies. Both 3+ and 4+ valence states of Pr are evidenced, irrespective of the host and calcination temperature, T (T = 500 and 1000 °C) with consequences on absorption, surface, vibrational and luminescence properties. Only zirconia represents a suitable host for Pr(3+) luminescence. The distinct trivalent Pr centers and their excitation mechanism are identified in relation to the tetragonal and monoclinic phases of ZrO2. A near-infrared to visible up-conversion (UPC) emission of Pr(3+) is observed upon excitation at 959 nm which occurs, most probably, via a two-photon excited state process. By using a multi-wavelength, time-gated excitation, the UPC process is established as phase selective, i.e. only Pr(3+) located in the monoclinic sites of the mixed phase, monoclinic and tetragonal ZrO2 (T = 1000 °C) contribute to the UPC emission. We believe that, besides the local symmetry, a key role in phase selective UPC is played by the presence of Pr(3+) low-lying 4f 5d levels. To the best of our knowledge, this is the first report of phase selective up-conversion emission in a lanthanide doped multi-phase host.

Electron Paramagnetic Resonance Characterization of Mn2+Ion in GaAs:Mn Crystal

The electron paramagnetic resonance (EPR) of a new Mn 2+ center in a GaAs:Mn single crystal has been investigated with an X-band spectrometer at 3.8 K. The spectroscopic splitting parameters g i j and the zero-field splitting parameters B k q were fully determined using the effective spin Hamiltonian. The local site symmetry of the Mn 2+ ion turns out to be monoclinic from the rotation patterns together with the spectroscopic splitting parameters and the zero field splitting parameters. The new Mn 2+ center with monoclinic site symmetry is believed to originate from interstitial Mn 2+ ions. The energy levels of the ground state for the Mn 2+ impurity ion embedded in a GaAs crystal, are calculated as a function of applied magnetic field.

Order and disorder effects in nano-ZrO2 investigated by micro-Raman and spectrally and temporarily resolved photoluminescence

Pure and europium (Eu(3+)) doped ZrO(2) synthesized by an oil-in-water microemulsion reaction method were investigated by in situ and ex situ X-ray diffraction (XRD), ex situ Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), steady state and time-resolved photoluminescence (PL) spectroscopies. Based on the Raman spectra excited at three different wavelengths i.e. 488, 514 and 633 nm and measured in the spectral range of 150-4000 cm(-1) the correlation between the phonon spectra of ZrO(2) and luminescence of europium is clearly evidenced. The PL investigations span a variety of steady-state and time resolved measurements recorded either after direct excitation of the Eu(3+) f-f transitions or indirect excitation into UV charge-transfer bands. After annealing at 500 °C, the overall Eu(3+) emission is dominated by Eu(3+) located in tetragonal symmetry lattice sites with a crystal-field splitting of the (5)D(0)-(7)F(1) emission of 20 cm(-1). Annealing of ZrO(2) at 1000 °C leads to a superposition of Eu(3+) emissions from tetragonal and monoclinic lattice sites with monoclinic crystal-field splitting of 200 cm(-1) for the (5)D(0)-(7)F(1) transition. At all temperatures, a non-negligible amorphous/disordered content is also measured and determined to be of monoclinic nature. It was found that the evolutions with calcination temperature of the average PL lifetimes corresponding to europium emission in the tetragonal and monoclinic sites and the monoclinic phase content of the Eu(3+) doped ZrO(2) samples follow a similar trend. By use of specific excitation conditions, the distribution of europium on the amorphous/disordered surface or ordered/crystalline sites can be identified and related to the phase content of zirconia. The role of zirconia host as a sensitizer for the europium PL is also discussed in both tetragonal and monoclinic phases.

A Variable-Temperature X-Band EPR Study of the Gd3+ Ion in a La2Si2O7 Crystal Characterized by Monoclinic Site Symmetry

Electron paramagnetic resonance (EPR) studies on a single crystal of diamagnetic compound La2Si2O7, potentially a phosphorescent/luminescent/laser material, with the Gd3+ ion substituting for the La3+ ion, were carried out at X-band (9.61 GHz) over the 4–295 K temperature range. The asymmetry exhibited by the Gd3+ EPR line positions for the orientations of the external magnetic field about the magnetic Z- and Y-axes in the ZY-plane was ascribed to the existence of monoclinic site symmetry at the site of the Gd3+ ion, as confirmed by the significant values of the spin Hamiltonian parameters g YZ , b 2 −1, b 4 −m (m = 1, 3), b 6 −m (m = 1, 3, 5), estimated by fitting all EPR line positions observed at room temperature for the orientation of the magnetic field in the magnetic ZX- and ZY-planes using a rigorous least-squares fitting procedure. At 8 K measurements were only carried out for orientation of B in the magnetic ZX-plane, due to difficulty in orientation of the crystal inside the cryostat, enabling estimation of all spin Hamiltonian parameters b n m except those characterized by negative m values and g YZ . The absolute sign of the zero-field splitting parameter b 2 0 was determined to be negative from the relative intensities of the lines at 8 K.

Extracting structural information from low symmetry crystal field parameters: Pr4+ in BaPrO3

Abstract Recent literature survey has revealed several crystal field parameter (CFP) datasets for rare-earth (RE) ions at orthorhombic, monoclinic, and triclinic symmetry sites, which cannot be directly compared. Triclinic CFPs include for a given rank ( k = 2, 4, 6) all components (− k ≤ q ≤ + k ). Depending on the axis system chosen, the low symmetry effects involved in the symbolic, fitted, or model CFPs may be only apparent and not actual . In order to extract useful structural information involved in the low symmetry CFPs, an approach comprising three methods is proposed: (1) the principal values of the 2nd-rank CFPs and the orientation of their principal axis system (AS) w.r.t. the original or crystallographic AS; (2) the pseudosymmetry axes method; (3) the closeness factors. This approach is applied to reanalyze the CFPs for Pr 4+ in BaPrO 3 .

Low-temperature ageing degradation of 2.5Y-TZP heat-treated at 1650 °C

Low temperature ageing degradation at 250 °C in air of 2.5 mol% Y-TZP with various grain sizes has been investigated by using specimens in the form of discs of 8 mm diameter and 1 mm in thickness. The increase in tetragonal grain size, as well as some cubic phase, were attained by heat-treating 0.3 μm fine grain size Y-TZP at 1650 °C for different times. XRD and Raman spectroscopy analysis were used to evaluate the amount of tetragonal to monoclinic (t–m) phase transformation after different ageing times at 250 °C in air up to 900 h and its effect on strength was measured by using the biaxial ball-on-three-balls test. It is shown that transformability under ageing conditions at 250 °C increases with tetragonal grain size. Raman analysis was done punctually at various distances form the edge of the aged samples and it is shown that the nucleation of monoclinic sites begins at the edge of the samples and that the depth of the transformation zone varies with the distance from the edge to the centre of the samples. Biaxial strength always decreases with ageing time and is influenced by the presence of micro and macrocracks formed at the surface centre of the aged samples. This depends strongly on the tetragonal grain size and on the depth of the transformed layer at the central part of the discs.

Local symmetry of the crystal-field Hamiltonian ofCePtSnby polarized neutron scattering

We present the results of a polarized neutron scattering study of crystal-field (CF) excitations in a $\mathrm{CePtSn}$ single crystal and their interpretation, describing the magnetic properties of the metallic Kondo antiferromagnet $\mathrm{CePtSn}$ in its paramagnetic phase. Using neutron polarization analysis, we demonstrate that all terms of the monoclinic site CF Hamiltonian have to be considered in order to account for all the available experimental data, including susceptibility and high-field magnetization measurements. The CF eigenstates, as probed by inelastic polarized neutron scattering, are not affected by cooperative phenomena neither in the vicinity of the Kondo temperature, ${T}_{\mathrm{K}}$, nor below the N\'eel temperature, ${T}_{\mathrm{N}}$.

Variable temperatureX-band EPR ofGd3+inLaNbO4andPrNbO4crystals: Low-symmetry effect, influence of host and impurity paramagnetic ions on linewidth, and onset of antiferromagnetism

EPR studies on ${\mathrm{Gd}}^{3+}$-doped diamagnetic ${\mathrm{LaNbO}}_{4}$ and paramagnetic ${\mathrm{PrNbO}}_{4}$ crystals containing ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ impurity Kramers ions were carried out at the X band (9.61 GHz) in the 4--295 K temperature range. The observed asymmetry in the extrema of ${\mathrm{Gd}}^{3+}$ EPR line positions in the angular variations of the EPR lines in the two hosts for the various transitions observed for the orientations of the external magnetic field about the Z and Y magnetic axes in the ac crystallographic plane was ascribed to the existence of the low, monoclinic site symmetry at the ${\mathrm{Gd}}^{3+}$ ion. The ${\mathrm{Gd}}^{3+}$ spin-Hamiltonian parameters were estimated from EPR line positions in the two crystals at 8, 73, and 295 K. The variation of the ${\mathrm{Gd}}^{3+}$ EPR linewidth as a function of temperature in the paramagnetic ${\mathrm{PrNbO}}_{4}$ host between 100 and 295 K is explained to be due to the dynamical dipolar and exchange interactions between the impurity ${\mathrm{Gd}}^{3+}$ ions and the host paramagnetic ${\mathrm{Pr}}^{3+}$ ions. The increase in the linewidth with decreasing temperature below 30 K in the two crystals implies the onset of antiferromagnetic ordering, consistent with ${T}_{c}=1.6$ and 2.4 K in ${\mathrm{LaNbO}}_{4}$ and ${\mathrm{PrNbO}}_{4}$ crystals. The observed decrease in the integrated intensity of ${\mathrm{Gd}}^{3+}$ EPR signal with lowering temperature in the two crystals is ascribed to the coupling of ${\mathrm{Gd}}^{3+}$ ions to the impurity ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ Kramers ions present in the samples.

Stability of monoclinic and tetragonal zirconia at low oxygen partial pressure

We have found that both tetragonal and monoclinic zirconia annealed at temperaturess in the range of 1100 to 1300 °C in atmospheres of low oxygen partial pressures (down to 10−26Pa) transform slowly to an apparently cubic phase. The transformation can be reversed by increasing the oxygen partial pressure sufficiently, i.e., exposing the sample to air again. These observations were made by111In/Cd perturbed angular correlation (PAC) measurements of undoped zirconia samples. Upon annealing under various reducing atmospheres PAC spectra show a steadily increasing fraction of Cd probe atoms in a locally cubic environment with the fraction of probe atoms in tetragonal or monoclinic sites decreasing accordingly.

EPR of Gd3+ in Sulfate Host Crystals of Low Symmetry

Abstract Electron paramagnetic resonance (EPR) spectra of trivalent gadolinium have been investigated in orthorhombic sulfate host crystals MeSO4 with Me = Sr, Ba, Cd, Hg, Pb and K2SO4 . Because of the low symmetry of the host crystals and the spin quantum number 7/2, complex spectra for several orientations of the magnetic field containing a large number of individual lines had to be analyzed. The spin Hamiltonian parameters have been rigorously determined from the rotational diagrams. Gd3+ has been found to substitute, first of all, for the monoclinic sites of the host metal ion with charge compensation at larger distances. To a small extent, charge compensation in the vicinity has also been observed, arising from additional centres and spectra with lower intensities. Correlations between EPR spectra and crystal structure are reflected in the values of the zero field splitting parameters.

The magnetic states of in

We have investigated the compounds and with the aim of establishing the magnetic energy levels and wavefunctions of the and ions. We report neutron inelastic scattering, specific heat capacity and susceptibility measurements, and analyse the data in two stages to obtain a model for the crystal field acting on the and ions. First we find the best model for the crystal field assuming tetragonal symmetry, and then we refine this model to include terms describing the actual monoclinic site symmetry . By this process we obtain a good description of the available data. We find that the low-temperature magnetic properties of in are dominated by two singlets, separated in energy by 4.3 meV. The level scheme consists of five, roughly equally spaced doublets, the ground and first excited states separated by 7.1 meV. The doublets are split by an effective magnetic field from the ordered moments, and the size of the splitting of the ground-state doublet is 1.5 meV.

Monoclinic Spin Hamiltonian Analysis of EPR Spectra of Mn2+ in BiVO4 Single Crystals

AbstractX‐band EPR spectra of Mn2+ in ferroelastic BiVO4 single crystals at room temperature taken in three orthogonal planes are analyzed using a twestage least‐squares fitting procedure. The absence of site splitting from the spectra shows unequivocally that Mn2+ lies on a site with two‐fold rotation symmetry, whose two fold axis is parallel to the monoclinic crystal axis. The low symmetry effects arising from the monoclinic site symmetry are evidenced by the observed coincidence of the extrema for transitions between different Zeeman levels along the monoclinic axis, and by the 180° rotational symmetry in the monoclinic plane and a small noncoincidence of the turning points in this plane. In the first stage, the spectra are fitted using the monoclinic form of the electronic Zeeman and fine structure terms (of second and fourth degree) appropriate for the crystallographically determined orientation of the monoclinic axis. In the second stage, fitting of the nuclear hyperfine tensor (A) and the nuclear quadrupole tensor (P) is carried out with fixed values of the electronic Zeeman tensor (g) and fine structure terms, i.e. the tensor (D) and the fourth‐degree terms, as obtained in the first stage. The principal axes of g, D, A, and P are found to be non‐coincident in the monoclinic plane, as would be generally expected for a monoclinic site.

Crystal Structure and Polarized Electronic Absorption Spectra of NaCo 2(SeO 3) 2(OH)

The crystal structure of hydrothermally synthesized NaCo 2 (SeO 3) 2(OH) has been determined by direct and Fourier methods using single crystal X-ray diffraction data up to sin θ/λ = 0.8 Å -1 [orthorhombic, space group Pbnm, a = 8.338(1) Å, b = 13.268(3) Å c = 6.042(1) Å Z = 4; R w = 0.023 for 1407 unique reflections with F o > 2σ(F o)|. The structure of NaCo 2(SeO 3) 2(OH) consists of [4 + 2]-coordinated Na atoms, distorted CoO 6 octahedra, pyramidal SeO 3 groups, and hydroxyl groups. The CoO 6 polyhedra form 1 ∞ [Co 4O 14] 20- zigzag chains, analogous to the respective Me 2+ chains in the olivine structure type. These chains are connected via the selenite groups and sodium atoms. The O atom of the hydroxyl group belongs to three CoO 6 polyhedra and forms a bifurcated hydrogen bond (3.312 Å, 2×). Furthermore, polarized electronic absorption spectra of NaCo 2(SeO 3) 2(OH) have been measured in the range from 4000 to 40,000 cm -1 using microscope spectrometric techniques. The spectra are characterized by electric dipole transitions at the acentric Co 2+ site [Co(2)]. An interpretation of the band splitting and polarization behavior is attempted in terms of a pseudotetragonal field component and the imposed actual monoclinic site pertubation, yielding 4 A 2 (F) and 4 A″( F) as respective ground states. Tetragonal crystal field calculations give Dq = 630, Dt = -135, Ds = -111, B = 843, and C = 3520 cm -1, which corresponds to a "cubic" Dq of 709 cm -1. The sign and magnitude of the tetragonal parameter Dt is in good agreement with the pseudotetragonal compression of the Co(2)O 6 polyhedron.

Analysis of single-crystal Mossbauer data in low-symmetry 57Fe centres

The Mossbauer single-crystal experiment is discussed with particular reference to the low-symmetry resonant sites. It is demonstrated that ambiguities associated with multiple, symmetry-related resonant sites (that is, sites whose Laue class is lower than that of the host crystal) are resolvable provided the absorber recoilless fractions can be precisely measured. A general computer program, MOSREF, is described that enables simulation of single-crystal spectra and/or refinement of Mossbauer parameters for a 57Fe centre in a site of any known symmetry in a host crystal of any known symmetry. The procedures are illustrated for a couple of ideal examples: a triclinic site in a monoclinic crystal and a monoclinic site in a trigonal crystal. Applications to recent experiments with the sheet micas biotite and muscovite are given.