We study the effect of crystal growth atmosphere (argon or air) and chemical doping (iron) on the quality of the single crystals with the general formula NdMn_1-xFe_xO_3+δ, which were grown by the optical floating zone method. The grown ingots were characterized in the manner of X-ray powder diffraction (XRPD), X-ray Laue diffraction, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), iodometric titration and single crystal neutron diffraction.

Single crystals of NdMnO_3+δ are usually grown following two main sets of preparation conditions:  1) ambient air atmosphere, rotation of 60 rpm and growing speed 10 – 20 mm/h [1] or 2) argon atmosphere of pressure 6 to 8 bars with 1 to 5 % of oxygen impurity, rotation of 40 rpm and growing speed of 6 – 8 mm/h [2]. Since both sets of preparation conditions might have the consequences on the oxygen nonstoichiometry δ [3] and thence on the physical properties of the prepared samples, or can lead to the decomposition of the material [4], a comparative study of  crystal growth conditions with respect to crystal grown atmosphere is important. That is why we have prepared several rods of NdMnO_3+δ with different preparation conditions (see Table 1) and have studied the prepared crystals by different characterization techniques.

The XRPD experiments revealed that the rods prepared in the air atmosphere are clean from the impurities, but the rods prepared in the argon atmosphere were free from impurities only at the beginning of the rods, but at the end the small fraction (about 2 %) of MnO impurity was detected. This impurity is probably caused by the evaporation of oxygen from the melt which resulted to the reduction of some manganese atoms to oxidation state +2. The subsequent EDX analysis of samples from trials 3 to 5 revealed that the samples are clean from impurities within the precision of the EDX technique (0.5 %).

The iodometric titration experiments resulted to the oxygen nonstoichiometry δ = 0.04; 0.14; 0.13; 0.19 and 0.02 for trials 1; 2; 3; 4; and 5 respectively.

The check of the grown ingot by Laue diffraction (trials 1 and 2) revealed the twinning of the crystals. The single crystal neutron diffraction experiments (trials 3 – 5) revealed that in the bulk material there are two types of grains: one type of grains grows in the manner that (101) axis is parallel to the growing direction, but in the same grown ingot there exist also the grain with the (020) axis parallel to the growing direction (see figure 1).  Fit of the neutron single crystal data revealed that the best crystal from the crystallographic point of view was crystal from trial 5, where the full width at half maxima (FWHM) for grain (101) was 0.76° and FWHM for (020) grain was 0.7°.

 
Figure 1: The omega scans as appear on the 2D neutron detector a) trial 3; b) trial 4; c) trial 5, data summed through the omega; d) trial 3; e) trial 4; f) trial 5, data summed through out of the scattering plane component. The position of the middle of the detector was trial 3: 36°; trial 4: 38°; trial 5: 36°; the detector covers angle range of 15.3 degree in 2θ and ± 7.65 degrees above and below scattering plane (at distance 748 mm from sample).

Concerning the iron doping for NdMn_1-xFe_xO_3+δ compounds, we have prepared several samples up to x = 0.3. For these experiments we have used the following crystal growth parameters: air atmosphere, flowing 2 l/min, growing speed between 6 and 8 mm/h; rotation of 30 rpm. All samples from this series were proven by XRPD and EDX analysis to be single phased and with the expected chemical concentration within the experimental error. The subsequent iodometric titration experiments revealed the small excess of the oxygen δ = 0.04; 0.10; 0.07; 0.11 and 0.14 for x = 0; 0.1; 0.2; 0.25 and 0.3, respectively.

In conclusion the air atmosphere is better choice than the argon atmosphere for crystal growth of NdMn1-xFexO3+δ single crystals. The iron solubility in NdMnO3 is very good and it is possible to obtain the series of NdMn1-xFexO3+δ compounds at least up to x = 0.3.

[1] A. M. Balbashov, S. G. Karabashev, Ya. M. Mukovskiy and S. A. Zverkov, J. Cryst. Growth 167 (1996), 365-368
[2] D. Prabhakaran, A. I. Coldea, A. T. Boothroyd and S. J. Blundell, J. Cryst. Growth 237 – 239 (2002), 806 – 809
[3] V. A. Cherepanov, L. Yu Barkhatova, A. N. Petrov and V. I. Voronin, J. Sol. Stat. Chem. 118 (1995), 53 – 61
[4] N. Kamegashira, Y. Miyazaki and Y. Hiyoshi, Mater. Chem. & Phys. 10 (1984) 299-304

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1. Preparation of NdMn_1-xFe_xO_3+δ single crystals – effect of preparation atmosphere and iron doping