Use a micrometer screw.

Use a micrometer screw.

Francesco, try TMA (Thermal Mechanical Analyzer) furnished with a flat tip probe. Apply minimal reasonable load (say 5-10 mN) to the probe at room temp. Good TMA models (e.g.  Perkin Elmer) easily provide 10 nm resolution in sample thickness measurements. Else, by running a simple temperature program you'll be able to measure the thermal expansion coefficients for your crystals (or phase transitions if any). Good luck!

Use a confocal microscope. Thats quick, exakt, not touching, no temperature influence on the sample.

 You have a mass and surface area, and density too. Thickness is calculated automatically. Of coarse, possible errors must be calculated

You can try electron microscopy, if you can tilt the probe on it

Before an answer I have two questions:

With which precision do you need the thickness? Do you need in one point ore in many points (two surfaces are never parallel and the must not be flat at all). I work in Crystal optics for X-rays, so I know a bit about that.

I am a physicist and do not know K alum. does this material has a normal long name or a chemical formula?

We use an instrument to measure the thickness of silicon plates (up to 20 mm thickness) with a precision below 1 micrometer. Measuring time few seconds. It commercially available . It is laser based and senses the interfaces with a kind of michelson interferometer (Low Coherence Interferometric Sensor (LISE) from FOGALE nanotech (France) ).

But we also use an optical microscop with a good software and look at a side surface.

The crystals you describe seem large enough to be mounted on a spindle axis device on a conventional optical microscope and measured by using a graded eye-piece (which you have checked by a certified standard) to read off the thickness from an edge view directly.

Hi,

Provided that you have a first measurement with a precision of the order of 1 µm and that the sample is more or less transparent to a given lambda ray, you could use interferometry to improve the accuracy.

If you want a high accuracy measurement , you can try to study  the coefficient  of transmission of some LASER light through your sample while tilting it by successive angle steps. By considering the interferences between the light reflected by both faces you should, at least in theory, come back to the refraction index and the depth of the sample.

An alternative way, is to mount the sample in a transmission spectrophotometer and to study the variation of the transmission whith the wavelength in a region of the spectrum where there is no strong absorption.

Interferometry is a very accurate technic.

Good luck!

In my answer above I omitted to mention the obvious point, well addressed by Vincent Bourdin, regarding the precision (and accuracy, may I add) required. Given the dimensions of the crystals I assumed the measurements obtained by direct observation in an optical microscope (the most simple and readilyy available method), would be sufficient. I hope Francesco can comment on this and check the feasibility of my suggestion. 

Thanks a lot for all these useful suggestions!

Jürgen: I would need a precision (not so important the accuracy) in the micrometer range. I am not very interested in the accuracy (provided that a hypothetical systematic error remains constant) of the measurement because I would need to record growth or no growth of the (111) face of these crystals in an experiment. The formula 

You can try imaging ellipsometry, but maybe the lateral resolution is not high enough (down to 1 micron).

Best regards

Christian

Sometimes you can try microscopy- putting a crystal on the glass slide with the thin line (with a marker) at the top of a crystal and make a thin line on the glass substrate. Then with the use of micrometer screw of the microscope you can measure the thickness of the object. Contact for an advice at your place with the microscopists. If anybody has SEM around you -it can help as well.

Good luck

Helena

But microscopy has the same lateral limitation like imaging ellipsometry......

Yes-but if you do not have elipsometry device you can have microscopy at a hand

Hajnik