Two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2 :

phase transitions and AFM order as seen by NMR

NICPB, Tallinn, Estonia: R. Stern I. Heinmaa A. Kriisa E. Joon

NHMFL,Tallahassee, USA:

P. Kuhns A. Harter A. Reyes W. Moulton

Department of Chemistry, Clemson University, USA:

M. Kartin-Ulutagay X. Mo W. Queen S.-J Hwu

Physics Department, Miami University USA:

J. Clayhold

Na5RbCu4(AsO4)4Cl2

a

c

Cu

O

As

a

c

a

c

a

c

Cu

O

As

nearly tetragonal nearly tetragonal insulating insulating magnetic materialmagnetic material

The copper and The copper and oxygen ions form oxygen ions form square tetramer square tetramer units units Cu4O44

spin exchange is spin exchange is expected to be expected to be two-dimensionaltwo-dimensional

S.-J. Hwu et al., J. Am. Chem. Soc., 124, 12404 (2002)

NMR study ofNMR study of Na5RbCu4(AsO4)4Cl2

The experiments are performed at 8,45T and The experiments are performed at 8,45T and 14,1T14,1T ( (4K4K << TT << 300K300K))

We used powders and small single crystalsWe used powders and small single crystals On powders we employed MASOn powders we employed MAS (12K < T < (12K < T <

300K)300K) All attempts to see Cu NMR above TAll attempts to see Cu NMR above TNN have have

yet been unsuccessfulyet been unsuccessful We have measured We have measured 8787Rb, Rb, 3535Cl and Cl and 2323Na NMR Na NMR

spectraspectra

High-resolution magic-angle spinning (MAS) NMRMAS) NMR

SSampleample is rotated is rotated about about an axis oriented at an axis oriented at 54.74° with54.74° with respect to respect to BB00

Sidebands become Sidebands become insignificant when insignificant when spectral width is smaller spectral width is smaller than than rotation frequencyrotation frequency

Our rotation frequencyOur rotation frequency: : 20 20 ÷÷ 40 kHz 40 kHz

100 50 0 -50 -100 -150 -200

ppm

Resolved Na+ Sites in RT MAS NMR/14.1T, νrot=40kHz

2323Na MASNa MAS

200 100 0 -100 -200 -300 -400

ppm

40.7K

72.4K

85.6K

86.3K

87.4K

104.3K

106.3K

107.4K

139.0K

203.5K

294.5K

Na5RbCu4(AsO4)4Cl2: single crystal 87Rb NMR

(i) Direct observation of the transition into the AFM ordered phase

(TN = 14.3 K in B0 = 14.1 T)

(ii) The complicated low-T 87Rb NMR line shape suggests a complicated,

incommensurate (IC) ordered phase

(iii) A plane-wave modulated IC spin state?

Stern, et al., unpublished

-300 -200 -100 0 100 200

9.80 K

10.80 K

11.76 K

12.70 K

13.86 K

14.05 K

14.13 K

14.25 K

14.42 K

Shift from RbCl [kHz]

B0 = 14.1 T || [010]

0 50 100 150 200 250 3001600

1800

2000

2200

2400

2600

2800

87

Rb Quadrupolar Coupling

87νQ∝VZZ

[ ]kHz

[ ]Temperature K

0 50 100 150 200 250 300

50

100

150

200

250

10

20

30

40

50

satellite transition (3/2 ↔ 1/2) (-3/2 satellite transition↔ -1/2)

[ ]Linewidth of the satellites kHz

[ ]Temperature K

(1/2 central transition↔ -1/2)

[ ] Linewidth of the central transition kHz

ConclusionConclusion

0 50 100 150 200 250 300

Temperature [K]

PMLT1LT2AFM

11

Acknowledgements

NICPB, Tallinn, Estonia: R. Stern I. Heinmaa A. Kriisa E. Joon

NHMFL,Tallahassee, USA:

P. Kuhns A. Harter A. Reyes W. Moulton

Department of Chemistry, Clemson University, USA:

M. Kartin-Ulutagay X. Mo W. Queen S.-J Hwu

Physics Department, Miami University USA:

J. Clayhold