Electron paramagnetic resonance and proton electron-nuclear double-resonance studies of a hydrogen-compensated Fe + (S = -') center in synthetic iron-doped a-quartz have been carried out at 20 and 15 K, respectively. The spin-Hamiltonian parameters were determined, allowing anisotropy of the g matrix as well as [g, D, 3 {'H)]-matrix noncoaxiality, and including high-spin terms of the form S . Evaluation of the results gives strong evidence that the center (called S& by some workers) consists of a Fe + ion occurring substitutionally at a Si + site, charge compensated by an interstitial hydrogen ion. The label [Feo~/H+] is proposed for the center.

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The successive phase transitions of (NH,),H(SO,), crystal and its deuterized analog are studied from 90 to 420 K using V02+ ion and the SeOT radical as EPR probes. On the basis of EPR experiments with ferroelastic domain switching in phase 11, a new model of the VO2+ complex in the crystal is proposed, with V02+ occupying an NHt (I) vacancy and able, a t high temperatures, to reorient very extensively coordinating in three ways with four of the nearest six oxygen atoms. Gradual hinderin of NHI reorientation with decreasing temperature enables reorientations in the sublattice of SO!-i ons and is invoked as being the reason of all the transitions occurring below room temperature. Gesi's crystal model of two oppositely polarized NH: sublattices in phase 111, as well as the ferrielectric nature of the spontaneous polarisation of phases VI and VII, are confirmed. The V02+ complex line splitting in phase V I of the denterized crystal behaves similarly as the anomalous temperature dependence of spontaneous polarisation observed by Osaka et al.Unter Verwendung von V02+ -1onen und Se08-Radikalionen als EPR-Sonden werden die sukzessiven Phaseniibergange an Einkristallen von (NH4)3H(S04)2 und dem deuterierten Analogon im Temperaturbereich zwischen 90 und 420 K untersucht. Aufgrund der experimentellen EPRResultate zum Schaltverhalten ferroelastischer Domanen in der Phase I1 wird ein neues Model1 des V02+-Komplexes in diesen Kristallen vorgeschlagen. Es wird angenommen, da13 das V02+-Ion die Stelle des NHl(I)-Ions einnimmt, und da13 es bei hohen Temperaturen betrachtliche Reorientierungen ausfiihren kann, indem es auf drei Weisen mit jeweils vier von den sechs nachsten Sauerstoffatomen koordiniert. Die zunehmende Behinderung der Rotation der NHa-Ionen bei Temperaturabnahme ermoglicht Reorientierungen im Untergitter der SO?--Ionen und wird als Ursache aller Phaseniibergange unterhalb Raumtemperatur angesehen. Es wird sowohl das Gesi-Model1 des Kristalls, welches aus zwei entgegengesetzt polarisierten Untergittern von NHtIonen in der Phase I11 besteht, sowie die ferrielektrische Natur der Polarisation der Phasen V I und VII bestatigt. Die Linienaufspaltung der V02+-Komplexe in Phase VI des deuterierten Einkristalls weist ein Verhalten auf, welches der von Osaka et al. beobachteten anomalen Temperaturabhangigkeit der Polarisation ahnlich ist.

A new type of cation-compensated Fe + (S = z) center designated [Fe04/M]&, with M =Li and H, has been found to exist in synthetic iron-doped a-quartz, through an X-band electron paramagnetic resonance study. The spin-Hamiltonian parameters at temperature approximately 20 K were determined for both centers, allowing for anisotropy of the g factor as well as gand 0-matrix noncoaxiality, and including high-spin terms of the form S . While the Li hyperfine matrix was not obtainable, the EPR measurements did yield the proton hyperfine matrix for M =H. For M = Li, in contrast to the C2 symmetry of the previously studied [Fe04/Li], center P shows C, symmetry at low temperatures (20 K), but approaches C2 symmetry at higher temperatures (approximatelỹ 160 K). The hydrogenic center, designated as [Fe04/H]0&, shows C& symmetry up to room temperature. The analysis of the spin-Hamiltonian parameter matrices for both the n (the previously studied form) and the P form of the cation-compensated Fe + centers indicates that the (interstitial) M ion is located, respectively, at opposite sides of the slightly distorted [Si(Fe)04] "tetrahedron. "