The invention relates to an apparatus (1) for clamping flat samples (6), in particular pouch battery cells, for x-ray diffractometry, wherein the apparatus has a housing (2) having a sample holder (4), which has holding elements (5) that are able to be tensioned in relation to one another for clamping the sample (6), at least two x-ray windows (11a, 11b, 12) for letting in and out x-rays, and at least one first temperature control device (7) for controlling the temperature of the sample (6). At least one first temperature control device (7) is in each case attached to the holding elements (5), wherein the first temperature control devices (7) are thermally coupled to the housing (2), and the apparatus has at least one second temperature control device (9), which is configured to dissipate heat, which is output by the first temperature control device (7) to the housing (2), out of the housing (3) to the outside and/or to introduce heat from the outside into the housing (2).