Measuring Phonons in Crystals – Please Read Carefully!
Alfred Baron, 2010, 2016, 2022, 2023

The single most common point of failure of IXS experiments is insufficiently characterized orotherwise poorly prepared samples.

Any collaborator bringing a sample must insure it is the correct structure, single
domain, of good quality, with known alignment and mounted appropriately on a holder
compatible with BL instrumentation.
Below are some sample issue stories – these are real, and often involve experienced groups and goodscientists who made incorrect assumptions, or did not check the work by their students orcollaborators. If you have not done single crystal diffraction (with good results) on exactly thesample you intend to use for IXS, you can be setting up the IXS to fail.
1. Sample was checked with Laue diffraction and appeared OK. Found in IXS to have large tails,poor mosaic -> poor data, impossible to get near to gamma.
2. Sample was checked using single crystal diffraction for only c-axis reflections. Found to bepoorly ordered in ab plane while trying to set up for IXS -> IXS experiment impossible.
3. Sample had a complex structure that sample responsible thought they understood sufficiently
but really did not -> wasted 2 days of IXS time trying to understand the sample.
4. Sample was too thin. Scans that should have been 2-3 hours needed 8-10 hours and even thenthe data quality was poor.
5. Sample had residual flux on it, leading to strange backgrounds in a low-rate experiment at smallenergy transfer -> experiment failed.
6. Sample was “known” to have a particular structure but, after the experiment was found to havea different (related) structure and not to be the intended sample. No publication.
7. Sample was a film expected to have a particular structure. While setting up for IXS it was
found to be different than expected -> 3 days spent doing diffraction from the sample tounderstand it, 1 IXS spectrum only. After experiments sample determined to just be wrong.
No publication.
8. Sample from INS people known to be twinned and expt. planned based on equal domain
populations. But no twinning observed in IXS. Possibly better results, but expt. plan had tobe changed on the fly.
9. Sample prepared by a student from an experienced group, but the PI did not check the Lauepattern. Cut so that the desired IXS experiments were not possible. -> lost ~1 day of IXS
beamtime for re-cutting and polishing.
10. Sample looked pretty but not checked by diffraction. Extra phonon peaks observed indicating
either signficant impurity phase and/or multi-grain sample Data may be salvageable.
11. Sample mounted with known surface normal but azimuthal direction not checked – impossible
to measure at optimal momentum transfers in the cryostat.
12. Sample measured with what was called single crystal diffraction but was not really (just a thtth scan) – found to have at least two different domains.
The following are some relevant points for measuring phonons in single crystals using IXS. Note,especially, that a sample prepared for neutron scattering, and especially for inelastic neutron
scattering (INS), can sometimes be of poor quality for IXS measurements (the constraints are justdifferent for INS) – please be careful – see 7, below.

0. Laue/Transmission vs. Bragg/Reflection Geometry. One sometimes, especially whencutting a crystal from a large piece, can choose to use either a transmission geometry or a reflection
geometry (Laue and Bragg, respectively). Generally, if one knows the specific momentum transfer

of interest, a reflection/Bragg geometry will give higher rates (e.g. x2). However, a transmission
geometry will usually be more flexible, allowing access to more of reciprocal space. Of course, fora transmission geometry, the sample needs to be thin enough (see 2). Conversely, for a Bragg
geometry, the sample must be thick enough so that we do not see scattering from any supporting
material (e.g. the copper of the sample holder) behind the sample.

1. Larger (up to a few mm) is almost always easier. We can and do measure ~10 micron
samples. But count rate is always an issue, and aligning a small sample is often harder than aligninga larger one, and also, there can be beam shifts during scans, or sample shifts on cooling, etc. 1mm
samples are much easier, with, probably, ~2x2mm2 faces being close to ideal. If larger than ~5 mm(or thicker than 2 mm), consider cutting a smaller piece, or talk to a local contact, to make sure thesample will fit.

2. Know the attenuation length. You should know the absorption length of your sample.
This sets the scale for the size and possible interference/shadowing from surface features/roughness.
It also sets the length scale for a transmission (Laue geometry) measurement. For transmission
measurements, the sample should be 1 to 1.5 absorption lengths thick along the x-ray beam – and not
more than 2. You can use http://henke.lbl.gov/optical_constants/atten2.html to estimate theattenuation length – usual phonon setup operates at 23724 eV / Si (12 12 12) or 21747 eV/Si(11 1111) or 17794 eV (Si (999) for high pressure).

3. Pre-aligned samples are required. Knowing the alignment of the sample (i.e.: two nonparallel reflections, or two axes of the unit cell, etc, not just one surface normal) at the level of a few(say ±5) degrees is crucial. While we will refine the alignment on the spectrometer, to go searchingfor Bragg reflections from scratch is a waste of beam time.

4. The sample must be a SINGLE DOMAIN. Be sure your sample is a single crystal
without, say, some tiny domains with a different orientation (as can easily appear in flux-grown
crystals). The beam size is ~50 microns, and often does not illuminate the full sample, so if present,
micro-domains can appear unexpectedly and lead to strange results. You are urged to put your sample
on a 4-circle diffractometer and scan over ALL of symmetry allowed reciprocal space to insure nomicro-domains are present. Note: TWINNING, should be avoided as this can impact (severely!)
careful measurements.

5. Mosaic. The mosaic requirements for IXS are relatively relaxed by x-ray standards, unless youneed either very good Q resolution, or want to work very near a Bragg peak, etc. Typically, 0.1
degree mosaic is not a problem. We begin to worry above 0.5 degrees. Of course, smaller mosaic isusually better.

6. Azimuthal Orientation is Important. The azimuthal orientation of the sample (ie:rotation about the scattering vector) has nearly no impact on the momentum transfer measured by theanalyzer crystal in the center of the arm. However, it has huge impact on the momentum transfers
seen by the other analyzer crystals and so can be very important to make proper use of the analyzer
array (esp. for transverse modes). See the discussion in section 4.6 of http://arxiv.org/abs/1504.01098

7. FOR NEUTRON SCATTERERS. Please note well that relative to neutron
scattering, x-rays see only the surface of the sample (see 2 above). That surface should be flat andunstrained, and, ideally, lightly etched and/or carefully polished. Be careful: even if the sample wasgreat for neutron scattering, the surface might be of very poor quality for x-ray scattering.