EPR of Nd3+ ions in PbCl2 single crystals

Solid State Communications, Vol. 58, No. 12, pp. 877-879, 1986. Printed in Great Britain.

0038-1098/86 $3.00 + .00 Pergamon Journals Ltd.

EPR OF Nd 3÷ IONS IN PbCI2 SINGLE CRYSTALS A.G. Badalyan, P.G. Baranov and V.A. Chramtsov A.F. Ioffe Physics-Technical Institute, Acad. of Sci. of the USSR, Leningrad, USSR and ~. Barta and J. Rosa Institute of Physics, Czechoslovak Acad. of Sci., 180 40 Prague 8, Na Slovance 2, Czechoslovakia

(Received 14 February 1986 by VAI. Agranovich ) X-band EPR measurements of Nd a÷ doped single crystals of PbC12 have been performed at the liquid-helium teml~eratures. The spectrum can be described by the spin Hamiltonian ~ = ~.S.g.H + ~.A.], gx = 1.530, gy = 2.842, gz = 3.147,Ax = 439.7, Ay = 840.0, Az = 987.5 for 14aNda÷ and Ax = 271.9, Ay = 522.8, Az = 616.1 for t4SNda÷ isotopes respectively (all A in MHz). The Nd 3÷ ions substitute the Pb 2÷ ions in the PbC12 lattice. THE Nd 3÷ ION belongs to the rare earth group with 143Nda+ (8.3 and 12.17% of natural abundance)with three electrons in a unfilled 4f-orbital. Its ground state nuclear spin I = 7/2. is "19/2. A low symmetry crystal field splits this level The amplitude ratio of the lines of both group., into 5 Kramers doublets of which only the lowest one corresponds to the natural abundance of both isotopesl is filled at helium temperatures. This is the reason why similarly, the ratio of h.f. interaction constants equals the EPR spectrum can be described by Hamiltonian with to the ratio of nuclear magnetic moments of both S = 1/2 effective spin. odd isotopes (see Table 1) The PbC12 crystals were prepared by Bridgman 143A/145A 143/d/145~ = 1.609. method using very pure (7N) raw material. They do not contain both cation and anion-type impurity (O 2-, The above facts unambigiously demonstrate that the OH- etc). They were doped by neodymium (mol 0.1% in observed spectrum belongs to Nd 3÷ ions incorporated into the PbC12 lattice. the melt). The EPR spectra of the Nd 3+ ion can be described If the Pb 2÷ ion is replaced by the Nd 3÷ ion one by the spin-Hamiltonian positive charge must be compensated. One of the studied crystals was grown with a neodymium and potassium = ~" (gxHxSx + g~1-1ySy + gzHzS,) + admixture (NdCIa + KCI)- designated "compensated", the other one with the admixture of neodymium only A,~Sxlx + AySyl~, + AzSzI~, ("noncompensated"). In the latter case the compensation can proceed in various ways, e.g. by vacancies in Pb ~÷ where S = 1/2 and I = 7•2. The parameters of the above sites or by C1- ions in interstitial positions (viz. SrF2, Hamiltonian are given in Table 1. It is noticeable thal CaF2..'.). The method of compensation of the Nd 3+ the ratios ions by means of potassium leads to the more perfect real crystal structure and enable to grow much more Ax:Ay:Az = gz:gy:gz, bigger crystals. The EPR spectra were registered by means of a are retained for both isotopes with an accuracy of a few standard X-band spectrometer. The temperature of the percents. This fact demonstrates that no significan| samples was maintained with an accuracy up to "~ 0.5 K overlap of the states with different J takes place. (A,, within the range of 4.5 to 40 K. In Fig. 1 EPR spectrum the nonequidistantness of the h.f. structure lines is small of Nd 3÷ ion in compensated crystals is shown. It consists 2nd order effects were neglected in the calculation ol of one intense central line and two group of 8 lines each. the h.f. interaction constants [1 ] ). The position of Nd s÷ ion in PbC12 lattice can b~ The lines of each group have approximately identical intensity and they are distributed nearly equidistantly. studied by means symmetry properties of an angl~ The intense central line belongs to even Nd 3÷ isotopes; dependence of its EPR spectrum. Rotating the crysta the two groups of lines corresponding to hyperfine around the b axis (with magnetic field in ac planel (h.f.) splitting, caused by two odd isotopes t4SNd 3÷ and doubling of the spectra can be observed. The curves o: - -

877

878

EPR OF Nd a÷ IONS IN PbC12 SINGLE CRYSTALS •

Vol. 58, No. 12

PbCI~ : N d , K y

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Fig. 2. Angle dependences of EPR spectrum of Nd a÷ center in a PbC12:N~ K crystal. T = 12K; p = 9 . 1 1 4 GHz (a) He(a, c); (b) He(c, b).

b

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Fig. 1. EPR spectrum of Nd a÷ center in a PbC12 :Nd, K crystal. HIIz, HJa, b; T = 12 K; ~, = 9.114GHz. the angle dependence of both parts of the spectra (Fig. 2) can be transformed one to an other by the reflection through the a and c-axes respectively. These properties imply that the Nd 3÷ ion is located in the Pb 2÷ lattice site. The PbC12 crystals belong namely by their structure to D~6 symmetry group with lattice parameters a = 0 . 7 6 0 8 # m , b = 0 . 4 5 2 5 g m and c = 0.9030/am [2]. The point group of Pb 2÷ site is Cs containing two elements, viz., identity and reflection through the bc plane. These properties can explain above mentioned symmetrical properties of the EPR spectrum [3]. From angle dependence of the spectra in Fig. 2 can be seen further that the main x and y g-tensor

axes are rotated from the a and c crystal axes by 37 ° respectively the z axis being identical with the b one. Detailed analysis of the ions positions in the PbC12 lattice [3] indicates that the x axis is oriented in the direction of the nearest neighbour in the Pb 2÷ sublattice. In the case of the Ga a÷ the authors in [4] report the same angle, while for E u 2÷ report the angle 23.5 ° (in agreement with [3] ). The x andy axes form an angle of 6.5 ° with the axes x', y ' of the crystal field of a pure crystal (z' = b) [3]. Influence of charge compensation of the Nd 3÷ center was studied by comparing of the EPR spectra both the compensated and the noncompensated samples, respectively. The influence was manifested in the line width and by the appearance of new lines in the g ~ 2 region of the EPR spectrum of the noncompensated samples. Nevertheless, the values of both the g-factor and the h.f. interaction constants agree within the range of experimental error for both compensated and noncompensated samples. In the compensated sample the width of the central line is much smaller (Table 1) than in the noncompensated one. Yet, their temperature dependence differ only slightly: from 20 K the central line width of the two samples abruptly increases due to

Table 1 direction

X

Y Z

g

1.530 2.842 3.147

143A/145A

A MHz 14a Nd

14s N d

439.7 840.0 987.5

271.9 522.8 616.1

AHmT PbC12 : Nd, K

1.617 1.607 1.603

Note PbC12 : Nd

1.6

3.6

0.493

0.863

Z, yc = 37 ° b-axis

Vol. 58, No. 12

EPR OF Nd a÷ IONS IN PbC12 SINGLE CRYSTALS

strong reduction of the spin-lattice relaxation times. Increasing line width of the noncompensated sample may indicate e.g. increased interaction of the Nd 3+ electron with the compensating C1- ion the nuclear magnetic moment of which is greater than the nuclear magnetic moment of potassium nucleus in a compensated sample. A much more homogeneous compensation by potassium ions as compared with the compensation by own defects is indicated by the much more distinctly expressed h.f. structure in the compensated samples. On the contrary, the new lines appearing in the EPR spectrum of a noncompensated sample indicate a far more complicated compensation mechanism in this sample. • Explanation of the other nature of the new lines

879

with g ~ 2 in the EPR spectra of noncompensated PbC12 :Nd samples as well as interpretation of the mechanism of positive charge compensation in the Nd 3÷ center in the PbC12 lattice require further measurements.

REFERENCES I.

A. Abragam & B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford (1970); R.J. Elliot & K.W.H. Stevens,

Proc. Roy. Soc. A218, 553 (1953). 2.

Jul.

Nozik, L.E. Fykin & L.A. Muradyan,

Kristallografia 21,76 (1976). 3. 4.

Q.H.F. Vrehen & J. Volger, Physica 31, 845 (1965). H.C. Beijerinck & B. WiUemsen, Physica 47,515 (1970).