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EPR Studies of Cr3+ and Fe3 + in Host Crystals with Langbeinite

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EPR Studies of Cr3+ and Fe3 + in Host Crystals with Langbeinite EPR Studies of Cr3+ and Fe3+ in Host Crystals with Langbeinite Structure J. Kastner*, B. D. Mosel, and W. Müller-Warmuth Institut für Physikalische Chemie der Westfälischen Wilhelms-Universität, Schlossplatz 4/7, D-48149 Münster, Germany Z. Naturforsch. 52a, 1123-1133 (1997); received August 28, 1996 Electron paramagnetic resonance (EPR) spectra of trivalent chromium and iron ions have been investigated in host crystals of double sulfates with langbeinite structure, A2B2(S04)3, with + + 2 + 2 + A = NH4 , Rb , Tl and B = Cd , Mg . In most cases, single crystals could be grown, and the spin Hamiltonian parameters have been determined from the rotational diagrams. Powder spectra were analyzed and simulated as well. Three centres of axial symmetry with different zero field splittings have been observed for Cr3 + , and two for Fe3 + . The trivalent impurity ions have been shown to occupy divalent lattice sites substitutional^, but the environments are different depending on the structure and the mechanism of charge compensation. A qualitative description of the incorporation of dopants has been possible on this basis, taking into account the local relaxation and the ligand field stabilization effects. Temperature dependent single crystal and powder measure- ments have provided information on the zero field splitting and phase transitions. 1. Introduction Cr3 + , spin-orbit and spin-spin couplings lead to two 3 + Kramers' doublets with ms = ± 3/2, ±1/2; for Fe Double sulfates, isostructural with the mineral lang- there are three Kramers' doublets which split in the beinite K2Mg2(S04)3, have attracted considerable in- magnetic field into six Zeeman levels. Three and five terest for many reasons. Most compounds of this class allowed EPR transitions, respectively, can be ob- undergo structural phase transitions from a paraelec- served, whose resonances depend on the orientation of tric cubic phase (P2t3 symmetric) at room tempera- the crystal in the magnetic field. Considering the sym- ture to a ferroelastic orthorhombic phase (P212121 at metry of the sites of the paramagnetic ions in the low temperatures, sometimes via ferroelectric mono- langbeinites, the crystal field splitting of Cr3+ was clinic (P2t) and triclinic phases (Pl) [1-3]. Some of expected to be describable by one ZFS (zero field 3+ these materials exhibit electro-optic effects useful for splitting) parameter (ß2), that of Fe by four 3 the modulation of light at room temperature [4, 5]. parameters (ß2°, ß4°, ). The langbeinites can easily be doped with paramag- In the present paper we studied six compounds netic impurity ions such as Cr3 + , Mn2+ and Fe3+ to doped with Cr3 + and three doped with Fe3+ aiming a perform EPR studies. EPR spectroscopic data are systematic investigation of langbeinites with trivalent known first of all for Mn2 + in cadmium and magne- impurity ions. Information as incorporation, defect sium langbeinites [6-12], but scarcely and not system- structure, symmetry of various centres and charge atically for other impurity probes such as Cu2+ [13] compensation was expected. It should be possible to and V02+ [8]. The present paper is focused on triva- study relationships between spin Hamiltonian lent paramagnetic ions; to our knowledge Cr3+ has parameters, crystal structure and ionic radii. Temper- not yet been studied so far; our investigations of Fe3 + ature dependent data should give EPR information are based on a preceding paper of Böttjer and Stock- on phase transitions. hausen [14]. Cr3+ and Fe3+ have the configuration 3d3 and 3d5, respectively, and the groundstates are 4F and 6S. 2. Experimental 5 = 3/2 and S = 5/2 EPR systems are formed. For 2.1 Crystal Growth and Sample Preparation * Present address: Sachtleben Chemie GmbH, Dr.-Rudolf- Crystals of the double sulfates A2B2(S04)3 with + + + 2 + 2+ Sachtleben-Str. 4, D-47198 Duisburg. A = NH4 , Rb , Tl and B = Cd , Mg were Reprint requests to Prof. W. Müller-Warmuth. grown either by evaporation from aqueous solutions 0932-0784 / 96 / 1000-1123 $ 06.00 © - Verlag der Zeitschrift für Naturforschung, D-72072 Tübingen Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 1124 J. Kastner et al • EPR Studies of Cr3 + and Fe3 + in Host Crystals with Langbeinite Structure 1124 of the corresponding A2S04 and BS04 compounds or integration, correction and construction of rotational from the melt (Bridgeman-Stockbarger method). The diagrams [15]. procedures were similar to those described in [12]. The observed data were fitted to the spin Hamilto- Cadmium langbeinites could be obtained as single nian [16] crystals of sufficient size and quality; powder samples were prepared for all the materials. Special attention H = pBSgB+ X £ (1) had to be paid to Mn2+ impurities which could only n = 2 m = — n be avoided by extremely pure solutions. For that pur- with the Stevens operators Ö™ [17], the Bohr magne- 3+ 3 + pose, the crystals were doped with Cr and Fe ton pB and the magnetic field B.The principal values using solutions of elemental chromium (99.995%) and of the g tensor and the ZFS parameters B„m have to be iron (99.98%), respectively, in sulfuric acid free of derived from the spectra. The fitting program included manganese. The Cr3+ and Fe3+ concentrations are a matrix diagonalization followed by a least-squares between 0.05 and 2 mol%. Optimum amounts which fit of the field positions for the two rotations with the guarantee a sufficient signal-to-noise ratio of the EPR fitting parameters gx,gy,gz, B™ and the crystal orien- spectra without line broadening were tested experi- tation in the laboratory system (misorientation). mentally. The dopants did not influence the morphol- ogy. All those crystals doped with Cr3+ had to be annealed for several hours at temperatures between 3. Results 570 K and 630 K. 3.1 A Cd (SOJ :Cr3+ with A = NH , Rb, Tl Most crystals were found to grow with large (111) 2 2 3 4 faces. The samples were mounted adequately in the Single crystals were grown from all these materials EPR cavity and could be rotated about axes accord- and both rotational diagrams and powder spectra ing to the morphology of the crystal. Information were measured. Figure 1 shows an example of the an- could also be obtained from powder spectra since the gular dependences of the single crystal EPR spectra point symmetry of the defects appeared to be suffi- characteristic of these three compounds. The rota- ciently high. tional diagram about a* reveals that the magnetic z axis coincides with the threefold crystal (c* || <1H>) axis. From the diagram about c* it can be concluded 2.2 EPR Spectroscopy that there are three different paramagnetic centres The EPR mesurements were made on a conven- with axial symmetry, each consists of four symmetry tional Bruker X-band spectrometer (ESP 300) in the spectra. Two rotational diagrams of measurements range from 9.33 to 9.78 GHz depending on sample about axes perpendicular to each other are therefore and measuring device. The microwave frequency was sufficient to describe the EPR properties. determined using a piceine standard (g = 2.0041(1)). It can be furthermore recognized from the spectra The spectrometer was furthermore equipped with a that the zero field splittings are of the same order of NMR gaussmeter and a refrigeration system. Two Pt- magnitude as the Zeeman splitting in the magnetic 100 resistors were used for either driving the tempera- field. The strong magnetic field limit cannot be applied ture control system or measuring the temperature at to simplify the description of the EPR spectra. In the sample site. addition to the lines which originate from Cr3 + , in the Single crystals were studied at various orientations field range between 310-370 mT signals from Mn2 + by measuring the spectra in steps of 2° when rotating impurities are observable. These will not be consid- about an c*-axis parallel to the <111) direction and ered further. about an a*-axis perpendicular to this. The axes de- The orientation of the crystal was determined noted by an asterisk may differ slightly from the direc- within the fitting procedure, i.e. misorientations of tions of the real c- and a-axes. Small misorientations up to 1 or 2° in terms of the Eulerian angles were could be corrected in the course of the fitting proce- found and included in the simulation program. As dure. The whole procedure was computer driven and a result of the simulation, which is also shown in fully automated. The data were transferred to appro- Fig. 1, the ZFS parameters of Table 1 were obtained priate computers and further manipulated using vari- for the three axial centres. The intensity ratio of the ous home-made programs for the search of signals, centres Crj:Cr2 • Cr3 turned out to be 30:1:2. For J. Kastner et al • EPR Studies of Cr3 + and Fe3 + in Host Crystals with Langbeinite Structure 1125 ~r B0/ mT B0/ mT 600 -\ 600-^ 4004 400 -\ V'-'N.: v.'.jünii,-'.1. •, i, ' • 200 2004 i,!a::!!:ii!!:!!!i:i!r8i!!!5 !!::i!!:::!ii:!!: :P!!!:j23rS!:M3SSSS!!!:i •in;:: •jiiinliij, .j;i>i;....::;uiii "'"'••i, "l!'*1' : iüiiiii:!!!!!!!!l!!!!!i«i»^ 180 60 120 180 Angle / degree Angle / degree B0/ mT 6004 4004 200- — ••ii, itiliiHiliiM,. Ii. tl|l|!itiii( |l«:ia.|a|iiHiiiii „;•. "!!l!!!!iiiiii;«iiini:::::::!!iiiiiimi T 120 180 60 120 180 Angle / degree Angle / degree 3+ 3+ Fig.
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