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THE MAGNETIC PROPERTIES of METATORBERNITE JOSE M. DIAZ Depaftonento De Fisica, Universidad Nacional, Bogota, Colombia HORACIO A

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THE MAGNETIC PROPERTIES of METATORBERNITE JOSE M. DIAZ Depaftonento De Fisica, Universidad Nacional, Bogota, Colombia HORACIO A Canad.ion Mineralo gist Vol.23, pp. 643-646(198s) THE MAGNETIC PROPERTIES OF METATORBERNITE JOSE M. DIAZ Depaftonentode Fisica,Universidad Nacional, Bogota, Colombia HORACIO A. FARACH AND CHARLES P. POOLE, JR. Departmentof Physicsand Astronomy, Universityof South Carolina,Columbio, South Carolina29208, U.S.A. AssrRAcr with the chemical formula Cu(UOTPO)r.8HrO and the structure sketchedon Figure 1, crystallizesin the The electron-spin-resonancespectra arising from the tetragonal syslem, space group PL/nmm (Doo7), IICu in metatorbernite exhibit strongly anisotropiq g- with two formula units per cell. Its crystal structure factors and linewidths. The anisotropy is explarnedin terms (Frondelet al. 1956,Ross & Evans1964, 1965, Ross of the magnetic interactions, which are stong within the et al. 1964, Makarov & Tobelko 1960) has IICu planes containing the copper atoms and waker between enclosedin a squareplanar configuration, wilh HrO planes. the qroleculesproviding very short Cu-O distances(1.91 presenceof Keywords: metatorbernite,g-factor anisotropy, electron- A). these qhort distancesindicate the spin resonance,magnetic interactions. very strong covalent bonds bdtweenthe copper and oxygen atoms. We selectedfor study large, well-formed crystals SoMNa.a,rns of metatorbernite on display at the McKissick . Museum of The University of South Carolina. They Les spectresde r6sonnancede spin dlectronique prove- weretaken from the Wisemanmine in SprucePine' IICu nant du de la mdtatorbernite se caract€risentpar des Mitchell Co., North Carolina in June 1932. facteurs-g et des largeurs de raie fortement anisofiopes. structuresoftorbernite and metatorberniteare par The L'anisotropie s'explique les interactions magudtiques: mainly in the number of water les plans qui les de very similar, differing fortes dans contiennent atomes cuivre, number of water faible entre ces plans. molecules. In the latter the moleculesvaries between8 and 12. In order to deter- (Traduit par la R6daction) mine whether our samplesare torbernite or metator- bernite, we carried out a X-ray-diffraction exami- Motsdds: m&atorbemite, facteur-ganisotrope, r€sonnance nation (powder pattern). The resultswere compared de spin 6lectronique,interactions magn6tiques. with the data of Frondel et al, (1956)and Ross e/ ql. (1964). INTRODUCTIoN ESR Rnsulrs Metatorbernite contains uranium, copper and phosphorus. The copper is divalent and hence para- In this section we present the linewidth and magnetic. This property means that metatorbernite gyromagnetic ratio obtained at room temperature can be studied by electron-spin resonance (ESR). In with a Varian Model 4502ESR spectrometeroper- this article we present the results of such a study and ating at a frequencyof 92.2'0MHzusing a power level interpret these in terms of the crystal structure. The of 250 mW with 100 kHz modulation. green color of this mineral arises mainly from the A typical ESR spectrum for metatorbernite (Diaz presence of divalent copper. l98l) is presentedin Figure 2. The peak-to-peak linewidth dependson the orientation of the crystal, and varies between0.02 and 0.034 Tesla. Such an Srnucrunn aNt X-Ray Rssurrs effect is not unexpectedfor a layered structure of this type (Cheung& Soos1978, Huang & Soos1974). Metatorberiite belongs to the torbernite group of The calculated linewidth can be estimated from the minerals, which is characterized by the formula expressionof Anderson & Weiss (1953): A,+ (JO/YO)2.aH2O), where,4 represents a cation andXis arsenic or phosphorus. This group contains AH=2.3 G Fd I S(S+ l) l% x lo-e (l) phosphates and arsenates of uranium arranged in a laminar structure AOfiOq)"u. Metatorbernite, where 8 :9.27 x t0-24JlTeslais the Bohr magne- 643 644 THE CANADIAN MINERALOGIST which is significantly larger than the experimental 000 ~ 000 ~ values. This fact, together with the angular depen- dence of the linewidth and the layered structure of Z/e ~ 000 ~ 00 the mineral, suggests that we are dealing with a two- dimensional magnetic system, as will be explained 3/4 .0 .~.o in the next section. 0" The g factor is anisotropic and varies with the u orientation of the crY.$tal,between the values of 2.06 p for g perpendicular to the c axis and 2.38 for g along 1/2 oO~ ~ @ the c axis, as shown on Figure 3. Values of g in this o~o 0" o 0 0, range are typical of those reported by various inves- H20 tigators for divalent copper (Diaz et al. 1971). A 1/4 Cu crystal-field-theory calculation (Poole & Farach 1972) made by us gave gn 2.16 and g.L = 2.03, ~ ~ ~ 0, = ~@ which differ considerably from the respective 000 ~ o 00 experimentally observed limiting values of 2.38 and @ 000 ~ 000 2.06. This discrepancy may be explained by the I r I I I I strongly covalent character of the copper-oxygen o 1/2 3/2 2 bonds. y/b FIG. 1. Sketch of the locations of the various atoms in the crystal structure of metatorbernite. Projected in the yz LINEWIDTHS OF Two-DIMENSIONAL plane, the copper atoms in the interplanar layers are MAGNETIC SYSTEMS shown located within distorted octahedra formed by square planar array of oxygen atoms from water As mentioned above, each lICU is centred inside molecules in the xy plane and oxygen above and below a square of water molecules with very short Cu-o along the z direction. Two unit cells are shown. distances. These distances are so short that the copper-oxygen bonds are strongly covalent and the water molecules are highly polarized. The uranium atoms lie along the c axis directly ton, g = 2.2, S = 'h and p is the density of above and below the copper atoms, as indicated on spins/m3. Estimating p from the molecular weight Figure 1. As a result, there are nearby UOz groups (938) and the mass density 3.70 g/cm3, that provide possible paths for superexchange inter- actions between the copper atoms. This causes the lines to be narrowed in accordance with the follow- LlH= 96 x 10-4Tesla (2) ing expression (3) where J is the exchange-interaction frequency and LlHois the linewidth in the absence of exchange nar- rowing. More importantly, we can also see from Figure 1 of Ross et al. (1964) that the copper atoms lie in planes with alternate interplanar separations of 6.58 and 10.73 A. The four nearest-neighbor copper ~ atoms all lie in the same plane. This causes the dipo- 25 G lar and exchange interaction to be dominated by the planar structure. Each copper ion is square planar co-ordinated with four oxygen atoms, thereby providing paths for the superexchange interactions, which narrow the lines. It therefore seems reasona- FIG. 2. Electron-spin-resonance spectrum produced by the ble for the magnetic interactions to resemble those lICu atoms in metatorbernite, where 10 = 10-4 T. of a two-dimensional spin system. MAGNETIC PROPERTIES OF METATORBERNITE 645 2.40 . 2.36 . 2.32 . 2.28 9 2.24 . 2.20 . 2.16 . 2.12 . 2.08 . 2.04 0 10 20 30 40 50 60 70 80 90 100 110 ANGLE,DEGREE g FIG. 3. Dependence of the factor of IICu in metatorbernite on the angle if; relative to the c axis. Following Cheung & Soos (1978) we can make use 40 of the following expression for the linewidth of a 38 two-dimensional system 36 34 . CJJ- f.H(¥"T) = 32 . peT) (3 cos2¥,_l)2_Q(T) cos2¥,+ R(T) (4) CJJ :J 30 . <t 28 . is the angle between the direction of the where ¥' ~~26 . external magnetic field and the plane containing the :J: 24 IICUatoms, and the temperature-dependent coeffi- <3 . cients peT) and Q(T) are determined experimentally. 22 . At ambient temperature we evaluated these coeffi- 20 . cients from measurements made at three different 18 angles, ~nd this provided the following expression .. 16 0 10 20 30 40 50 60 70 80 90 100 110 ANGLE,DEGREE f.H = [-5.94 (3 cos2¥,-1)2 + 20.12 cos2¥, + 25.64] x 10-4 (5) FIG. 4. Dependence of the linewidth AH or distance between the two peaks of the line ESR spectrum of IICu in metatorbernite on the angle if;relative to the c This expression provided a good fit to the axis, where 10 10-4 T. experimentally measured angular variation of the = linewidth, shown on Figure 4; the experimental error in the measured linewidths is about 5070. nar distances. This provides the possibility that the copper atoms form a two-dimension magnetic sys- DISCUSSION tem that is exchanged-narrowed within the planes. Our experimental results demonstrate that this is the The structure of metatorbernite shows that the case. The narrowing of a line due to the exchange IICU atoms are located fairly close together in interaction has been. observed and discussed by planes that are separated by relatively large interpla- Huang & Soos (1974). 646 THE CANADIAN MINERALOGIST AcKNowLEDGEIvtENTS Huaxc, T.Z. & Soos,Z.G. (1974):Interchain exchange in nearly one-dimensionalCu(NH)aftCla. Phys, Two of us (HAF and CPP) thank the National Rev.R9, 49814984. Grant ISP-80-11451,for par- ScienceFoundation, Mexer.ov,E.S. & Tonsr,ro,K.I. (1960):The crystal tial support of this work. structure of metatorbernite. Sov. Phys. Dokl. 5, 221-223. Poor-p,C.P. & Faneqr, I{.A. (1972):The Theoryof REFERENcES MagneticResonazce. Wiley-Interscience, New York. ANpsRsor.r,P.W. & Wuss,P.R. (1953):Exchange nar- Ross.M. & EveNs,H.T., Jn.(1964): Studies of the tor- rowingin paramagneticresonance. Rev. Mod, Phys. bernite minerals I. The crystal structure of aber- 25,269-n6. nathyite and the structurally related compounds NH4(UO2AsOf.3H2O and K(HTOXUO2ASO)2. CsruNc, T.T.P. & Soos,Z.G. (1978):Theory of 6H"O. Amer. Mineral. 49, 1578-1602.
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