NMR study of 133 Cs in a new quasi-one-dimensional conducting platinate

1. NMR study of 133Cs in a new quasi-one-dimensional conducting platinate Ian Leatherbury Undergraduate student, University of South Alabama, Department of Physics, Mobile, Alabama COLLABORATORS: J. A. Alexander and A. A. Gapud Department of Physics, University of South Alabama A. P. Weber, L. Pham, and R. E. Sykora Department of Chemistry, University of South Alabama A. P. Reyes and P. Kuhns National High Magnetic Field Laboratory FUNDING: National Science Foundation (NSF) – Research at Undergraduate Institutions NHMFL: DoE, NSF, State of Florida

2. Overview • TCP: new quasi-one-dimensional platinate • Intro: platinates – KCP • Structure from room-temperature x-ray diffractometry • Anomalous R(T) not seen in KCP • Method: NMR on 133Cs • Quadrupole splitting: verify Cs in structure, align sample • T1 versus temperature • Key Findings, preliminary results • Verified Cs structure • Anomalous peak in T1(T) • Conclusion, further work

3. Platinates: KCP • Quasi-one-dimensional (Q1D) conductors • Parallel Pt chains • Peierls transition • Structural change leads to new gap between valence and conduction bands • Metal to insulator transition • K2[Pt(CN)4]Br0.303 . H2O (KCP) • Well known • Pt chain ringed by 4 CN • Adjacent Pt planes rotated 45o • Pt-Pt spacing: • Longitudinal – along chain: 2.89 Å • Transverse – between chains: 9.88 Å • 4 H2O rings each Pt-CN plane • K, Br separate Pt-CN-H2O chains. • Peierls transition temperature not observed (above room temperature?): already semiconducting • DC R(T): increases with lower temperature T From: Toombs, Physics Reports 40, No. 3(1978)181.

4. CN Pt CN New Q1D platinate: TCP • Cs4[Pt(CN)4](CF3SO3)2 • Structure from room-temp XRD • Like TCP, Pt chain ringed by 4 CN and adjacent Pt planes rotated 45o • Pt-Pt spacing (room temp): • Longitudinal: 3.03 Å (5% longer) • Transverse: 13.61 Å (38% farther) • Cs instead of H2O • Triflates instead of K, Br

5. New Q1D platinate: TCP • Cs4[Pt(CN)4](CF3SO3)2 • DC transport measurements • R(T): anomalous peak below 200 K • NMR: Preliminaries: • Objective: T1 vs. T • Unlike KCP, could not find Pt ! • Used 133Cs instead

6. 133Cs NMR: Toward T1: symmetry • I = 7/2: Nuclear quadrupole moment Q: interaction with local electric field gradient (EFG) • Spin states are perturbed • Resonance frequency νo split into 2xI frequencies νm = νo + mνQ , m = 0, 1, … (I-1/2) • Splitting depends on local symmetry of 133Cs • Expect: four νm spectra: • Each Pt/CN plane: 4 Cs w/ same local symmetry • Dependence on angle θbetween applied field and crystal symmetry axis: A C C’ A’ A’ • Each plane: 2 equivalent symmetry sites, A and A’ shifted 90o : two spectra • Adjacent plane: same sites but shifted 45o: C and C’: two spectra C’ C A

7. 133Cs NMR: Toward T1: symmetry Data: • four quadrupole spectra • versus orientation θ

8. 133Cs NMR: Toward T1: symmetry Modeling using Labview • Check consistency with with sample data: • Find sample orientation To match data, sample angle θ = 3.17 deg

9. 133Cs NMR: T1 vs. T: data • Fixed field and orientation • Above 150 K: T1 ~ 200 s • KCP: ~ 10-3 sec! • Below 110 K: T1 ~ 103 sec • KCP: ~ 102 sec, 4.2 K • Anomalous “peak” • KCP: monotonic increase with cooling • Centered around 121 K • T1 down to < 30 sec 121 K

10. 133Cs NMR: T1 vs. T: fitting • Power law • No peak, monotonic • T < 120K: gap Δ • ~ exp( - Δ/ kT ); Δ/k~265K • Normally valid at lower T • Few points (long T1!) • Critical fluctuations • (T – Tc)-β • T > 120K: Critical fluctuations and power law

11. Summary: Not at all like KCP… • TCP: new platinate, anomalous R(T < 200K) • NMR on 133Cs • Confirmed the configuration of Cs in TCP, via angular dependence of quadrupole spectra • T1 vs. T: anomalous “peak” at ~ 120 K, precipitous increase below 120 • Best fit: combination of critical fluctuations and energy gap • Structural transition? Doing cryogenic XRD…