²⁰⁷Pb NMR of PbZrO₃ and PbZr_1-xTi_xO₃ Solid Solutions

Abstract

PbZrO3 undergoes a phase transition to a paraelectric phase at 230°C. During this phase transition the unit cell changes from orthorhombic to cubic. Structural changes of PbZrO3 have been monitored using solid state NMR by measuring the variation in the 207Pb chemical shielding tensor as a function of temperature. The two distinct lead sites show rather different behavior as a function of temperature. The less shielded lead maintains an almost constant asymmetry parameter η = 0.2 from 0°C to 200°C while the more shielded lead resonance becomes more axially symmetric as the temperature is raised going from η = 0.24 at 0°C to [.eta] = 0.08 at 200°C. Powder pattern singularities become less distinct near the phase transitions, but the temperature dependence of the chemical shift tensor principal values remains continuous, and there is no evidence of an intervening higher-symmetry phase. At the phase transition, both resonances collapse into a single narrow line characteristic of a nucleus at a high-symmetry site. Results of a preliminary study of the PbZr1−xTixO3 solid solutions by 207Pb solid state NMR are also presented.

Fundamental workings of chemical substitution at the A-site of perovskite oxides— a 207Pb NMR study of Ba-substituted PbZrO3

“…In NMR, the signal with the less negative isotropic CS and larger CSA has previously been assigned to the lead site with the shortest Pb-O distance. 35,39 With the 2D-PASS pulse sequence (Fig. 2b S1 †), and we suggest that any deviations of our data can be attributed to temperature gradients in the sample configuration, which can result in frictional heating.…”

“…S4 †). The chemical shift of PZ has been reported to be temperature-dependent between 0 and 30 °C, 39 which is why we refrain from assigning too much importance to minor chemical shift changes in this work.…”

“…37 Hence, broad line shapes or complicated spinning sideband (SSB) patterns under magic angle spinning (MAS) are characteristic aspects. 38 Furthermore, the sensitivity of the chemical shift to the probe temperature is to be expected, 39 and the spin-lattice relaxation times for known lead perovskite oxides have been found to vary in a broad range between 1 s and 160 s. 35 While the effects of barium substitution on the global structure and phase transition temperatures of PbZrO 3 are reported in previous literature, rationalization of the chemical and structural reasons for this effect remained elusive. This fact has motivated our current search for a local structural origin of the changes brought about by barium, particularly in relation to the chemistry of this substituent in perovskite oxides.…”

“…37 Hence, broad line shapes or complicated spinning sideband (SSB) patterns under magic angle spinning (MAS) are characteristic aspects. 38 Furthermore, the sensitivity of the chemical shift to the probe temperature is to be expected, 39 and the spin–lattice relaxation times for known lead perovskite oxides have been found to vary in a broad range between 1 s and 160 s. 35…”

Frequency-Swept Ultra-Wideline Magic-Angle Spinning NMR Spectroscopy

“…Single-Step Acquisition. For moderate NMR linewidths, as, for example, found in lead zirconate (PbZrO 3 ), exhibiting two distinct 207 Pb sites with superimposed CSA patterns spanning ∼150 kHz at 11.74 T, 48 CPMG-MAS may still be performed using conventional rf-pulses. 14 Nevertheless, the application of WCPMG-MAS results in slight sensitivity enhancement at lower or identical rf-fields, without compromising the spectral resolution (see Figure S3).…”