Single crystals of BeO which have been irradiated with neutrons show an EPR spectrum with partially resolved hyperfine structure. An ENDOR study of the hyperfine interaction leads to the conclusion that the spectrum arises from centers which consist of an electron trapped at an oxygen vacancy surrounded by four Be nuclei. The measured hyperfine interaction constants are: Ai/h= 10.3 MHz, A u/h= 16.4 MHz, and (e 2 qQ)/h= 1.1 MHz for the Be nuclei not on the c axis; and Ai/h = 4.2 MUz,A u /h = 9.2 MHz, and (e 2 qQ)/ h= 1.5 MHz for the nuclei along the c axis. These values lead to |^(fBe) | 2 =0.13X10 24 cm" 3 for the electron wave function at the c-axis nuclei, and |^(rBe) | 2 =0.06X10 24 cm -3 for the nonaxial nuclei. The measured g value for H parallel to the c axis is 2.0030.
Measurements of the angular variation of the quadrupole splitting in the NMR spectrum of Be in single-crystal BeO are reported and give e qQ/h =39.4+0.2 kHz. The dependence of the field-gradient parameter q on an applied external electric field is investigated and interpreted using the phenomenological relation of the form e(4y)i~=pRO~E~+p ZT&&&~e&~, where the first term represents the constant-strain electric-field-induced change and the second term represents the piezoelectric contribution.The strain-induced part of~q is measured separately and found to be negligible compared with the part due directly to the electric field. We report an experimental value for R~o f +(1.3 +0.5) x10 cm . Calculations of R~o n the basis of ionic and covalent models of the BeO structure give -4. 1x10 and -8.2x10 cm", respectively, for the ionic and covalent contributions and a total calculated Rz~o f -1.2 x10~cm . It is thus unlikely that the ionic model can account for the observed Stark shift.A determination of the crystal c axis was made using the pyroelectric effect and this, together with the Stark-shift data, indicates that the sign of the field gradient is positive.
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