1. 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
Hello, my name is Annika Kriisa. I'm from Estonia. I work at the National Institute of Chemical Physics and Biophysics (NICPB) in the Laboratory of Chemical Physics. Chemical Physics and Biophysics is the only national laboratory in Estonia. It conducts mostly experimental research, in strong national as well as international collaboration.
For COST P16 there are two main groups contributing - NMR and FIR.
I am going to present you a part of my thesis work with NMR group on a novel low-dimensional antiferromagnetic oxide.

2. Na5RbCu4(AsO4)4Cl2 nearly tetragonal insulating magnetic material
The copper and oxygen ions form square tetramer units Cu4O4
spin exchange is expected to be two-dimensional
We are studying a nearly tetragonal insulating magnetic material with a remarkable crystal structure. It is a two-dimensional, layered compound with a square-planar arrangement of copper and oxygen ions. The copper and oxygen ions form square tetramer units Cu4O4, which are connected by the AsO4 bridging units to form nearly tetragonal two-dimensional layers, as illustrated in figure. Thus the spin exchange of this material is expected to be essentially two-dimensional in nature.
I will show you our NMR results obtained on nuclei outside the 2D planes: Na, Rb and Cl (näita struktuuris!)
S.-J. Hwu et al., J. Am. Chem. Soc., 124, (2002)

3. NMR study of Na5RbCu4(AsO4)4Cl2
The experiments are performed at 8,45T and 14,1T (4K < T < 300K)
We used powders and small single crystals
On powders we employed MAS (12K < T < 300K)
All attempts to see Cu NMR above TN have yet been unsuccessful
We have measured 87Rb, 35Cl and 23Na NMR spectra
NMR juures: on teisi atomaarselt tundlikke tehnikaid (x-rays, neutron scattering etc), NMR on lähikorra (local order, short range) suhtes erakordselt tundlik
Because there are several excellent NMR nuclei in its structure – 23Na, 87Rb, 35Cl, 23Cu and 75As, we used NMR spectroscopy to study this material. NMR is the only experimental solid state technique that enables the study of material at atomic level.
The experiments were performed at 8,45T and 14,1T in a temperature range 4K<T<300K. We used both powders and small single crystals. On powders we employed fast spinning (MAS) techniques at room temperature down to 12K.
Of those local probes, Cu is the most suitable because of its position in the crystal and strong coupling to the spins. However all attempts to see Cu in paramagnetic state have yet been unsuccessful, probably because there are several technical difficulties: the lines get very broad towards low temperatures and the relaxation is rather fast. That is why we measured Rb, Cl and Na NMR spectra.

4. High-resolution magic-angle spinning (MAS) NMR
Sample is rotated about an axis oriented at 54.74° with respect to B0
Sidebands become insignificant when spectral width is smaller than rotation frequency
Our rotation frequency: 20 ÷ 40 kHz
A number of methods have been developed and considered in order to minimize large anisotropic NMR interactions between
nuclei and increase Signal to Noise ratio in NMR spectra. One method is Magic-angle spinning. In magic angle spinning
the sample is rotated about an axis oriented at 54.74°degrees with respect to the external magnetic field. As long as the rotation frequency is smaller than the spectral width, the spectrum during rotation consists of a line at the isotropic chemical shift and a pattern of sidebands that are spaced
by the rotation frequency. The intensity distribution of the sidebands still contains information on the chemical shift anisotropy tensor. If the rotation frequency exceeds the anisotropy, the intensities of the sidebands becomes insignificant and only the isotropic line persists. In practice, rotation frequencies larger than 20 kHz are usually required. At RT our rotation frequency was about 40kHz and at very low temperatures it was 20kHz.

5. As long as the rotation frequency is smaller than the spectral width, the spectrum during rotation consists of a line at the isotropic chemical shift and a pattern of sidebands that are spaced
by the rotation frequency. The intensity distribution of the sidebands still contains information on the chemical shift anisotropy tensor. If the rotation frequency exceeds the anisotropy, the intensities of the sidebands becomes insignificant and only the isotropic line persists. In practice, rotation frequencies larger than 20 kHz are usually required.

6. Resolved Na+ Sites in RT MAS NMR/14.1T, νrot=40kHz
100 50
-50
-100
-150
-200
ppm
Resolved Na+ Sites in RT MAS NMR/14.1T, νrot=40kHz
Here you can observe the Na sites in RT MAS NMR spectrum.
Na has 3 different sites or local positions in the crystal.

7. 200
100
-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
23Na MAS
We observed two phase transitions, which were not detected before.
At 110K there seems to be slight structural change (low-Temperature x-ray studies are currently being performed). At 84K we observed line splitting indicating the appearance of some local fields at Na and Rb sites. We beileve that at 84K the tetramers start ordering (short-range, intra-tetramer correlations only) until at 14,8K inter-tetramer long-range coherence will be created. So at 14.8K the whole crystal becomes antiferromagnetic.

8. Na5RbCu4(AsO4)4Cl2: single crystal 87Rb NMR
B0 = 14.1 T || [010]
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?
The Rb NMR spectra show a direct observation of the transition into the AFM ordered phase. The complicated double-horn type line shape with a continuum of finite intensity between peaks is a signature for an incommensurate structure. We notice that the spectra are not symmetric. This can be explained by the position of the 87Rb in the crystal or for example by the field being in cycloidal spin plane.
Stern, et al., unpublished

9. Quadrupolar satellites are giving direct information about the electric field gradient at the nuclear site
Without any phase transitions they should change monotonously with temperature. Changes in T-dependences around 110K and discontinuity below 80K support our findings of phase transitions

10. While the width of the central line(sensitive to quadrupole interaction only in second order) at half maximum seems to be temperature independent for temperature range 70K<T<125K, the sizeable broadening of the line width of the satellite transitions(where quadrupole interaction dominates) is observed at the same temperature range. This may indicate structural transition.

11. Conclusion T 74 110 Temperature [K] PM LT1 LT2 AFM N 50 100 150 20050
100
150
200
250
300
Temperature [K] PM
LT1
LT2
AFM T N 74
110
We detected two new phases of Na5RbCu4(AsO4)4Cl2 with boundaries at T≈74K±1K and T*=110±5K resulting in a phase diagram
Conclusion: We detected three phase transitions of Na5RbCu4(AsO4)4Cl2 with boundaries at Neel temperature TN=14,8K, T≈74K±1K and T*=110±5K resulting in a phase diagram.
The nature of LT1 (low temperature) and LT2 needs further studies.

12. Acknowledgements NICPB, Tallinn, Estonia: NHMFL,Tallahassee, USA:
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